Direct answer
For the Samsung Galaxy S25 Ultra, one network, all networks, VPN/profile and radio hardware produce different fixes. First, test the same network on another device and this device on another known network.
What this covers
- Samsung Galaxy S25 Ultra (SM-S938*)
- Samsung Galaxy family; S Pen, foldable display, DeX, USB moisture and One UI features vary by exact model.
- The specific fault this record addresses: will not connect to Wi-Fi or keeps disconnecting
What it does not cover
- ×The same symptom on a different Samsung product line, which has its own record and its own cited source
- ×An on-screen error code whose meaning has not been checked against this exact model's manual
- ×Live electrical, gas, sealed-system, internal battery-cell or structural repair — a technician boundary, not a self-service step
Differential
What actually causes this, and how to tell them apart
These are ordered by how often each one is the answer, not by how easy it is to try. The point of the middle column is that two causes producing the same headline symptom rarely produce the same detail — that detail is what tells you which one you have before you change anything.
- 1
The network, not the phone
What separates it Other devices on the same network have trouble too, or the problem started when the router was replaced or updated. The phone is the thing you are holding, which is why it gets blamed.
How to confirm Test another device on the same network at the same time and place. This costs nothing and immediately halves the problem.
- 2
The saved network entry has gone stale
What separates it It connects to other networks fine and fails on one specific saved network, often after a password change or a router replacement. The phone holds credentials that no longer match.
How to confirm Forget that network and rejoin it fresh. This is the highest-hit-rate single action on this symptom and it costs one password re-entry.
- 3
Band steering between 2.4 and 5 GHz
What separates it It drops briefly and reconnects, repeatedly, in a spot roughly halfway between two bands' usable range. A router advertising both bands under one name moves clients between them, and the handover is not always clean.
How to confirm Look at which band the phone reports being on when it misbehaves. Temporarily separating the bands settles it in one test.
- 4
Private or randomised MAC address against network filtering
What separates it It joins other networks and is rejected by one that uses MAC filtering, or by a workplace or campus network. Both platforms randomise the hardware address per network by default.
How to confirm Check the private-address setting for that specific network. On a filtered network the address has to be the real one or the randomised one has to be registered.
- 5
A captive portal that never completed
What separates it Connected with no internet, on hotel, café, campus or transport Wi-Fi. The sign-in page did not appear or was dismissed.
How to confirm Forget the network and rejoin so the portal is presented again. A saved captive network frequently reconnects without ever offering the page.
- 6
Marginal signal rather than absent signal
What separates it It works in one room and degrades in another, worse when doors are shut. A weak association drops throughput long before it drops the connection.
How to confirm Compare behaviour beside the router against the problem location. Consistent behaviour by place is signal; inconsistent behaviour anywhere is not.
- 7
The router's client table or DHCP pool is exhausted
What separates it The phone connects and gets no address, or drops when other devices join. Consumer routers have finite limits that produce no message.
How to confirm Check the router's client count against its limit, and restart the router to clear stale leases.
- 8
A VPN or private-relay feature interfering
What separates it Connected with a good signal and no working internet, on every network. A VPN that has failed open, or a privacy relay, sits between the phone and everything.
How to confirm Disable the VPN or relay and retest. This is a phone-side cause that looks exactly like a network fault.
- 9
Wi-Fi assist handing off to cellular and back
What separates it It appears to leave Wi-Fi in a weak-signal spot and data usage rises. The phone is deliberately switching to cellular when Wi-Fi is poor.
How to confirm Check whether the switch-to-cellular assist setting is on. This is a feature behaving correctly rather than a Wi-Fi failure.
Before you touch a setting
Record this first — changing settings destroys the evidence
Every item below stops existing the moment a reset, re-pair or settings change is made. Written down first, they are what separates the causes above; recovered afterwards, they are guesswork.
- Whether other devices on the same network are affected at the same time and place — the single most informative fact on this symptom.
- Whether it fails on every network or on one specific network, which separates a phone problem from a network one.
- Whether it says connected with no internet, or fails to connect at all. Those are different failures at different points.
- Which band the phone reports being on when it misbehaves.
- What changed and when: a router replacement, a firmware update, a password change, a new phone update, a move.
- Whether the problem is location-dependent within the building.
- Whether a VPN, private relay or content filter is active.
Test another device on the same network, same place, same time
ReversibleThis halves the problem in thirty seconds. Other devices failing too makes it a network fault and everything phone-side is wasted effort; other devices fine makes it the phone and the router is exonerated.
Establish whether it is one network or all networks
ReversibleOne specific network failing while others work points at that network's saved entry, its filtering or its portal. Every network failing points at the phone's radio, a VPN, or a system setting. These share no further steps.
Forget the network and rejoin it fresh
Signs you outThis is the highest-hit-rate single action available here. A saved entry holds credentials, band preferences and portal state that can all go stale after a password change or a router replacement, and rejoining costs one password entry.
Separate 'connected with no internet' from 'will not connect'
ReversibleConnected with no internet means association and authentication both succeeded, so signal, password and band are all excluded — look at the portal, DNS, a VPN or the router's own connection instead.
Check the private-address setting for that network
ReversibleBoth platforms randomise the hardware address per network by default, and a network using MAC filtering will reject the randomised one. This is invisible unless you look for it, and it explains a phone that works everywhere except one place.
Disable any VPN or private relay and retest
ReversibleA VPN that has failed open sits between the phone and everything, producing a strong Wi-Fi signal with no working internet on every network. It is a phone-side cause that looks exactly like a network fault.
Compare behaviour by location within the building
ReversibleConsistent trouble in one room and none beside the router is signal, and the fix is placement or a mesh node. Trouble that follows you everywhere is not signal, and no amount of router repositioning will change it.
Restart the router before resetting the phone
Signs you outStale leases and an exhausted client table both produce a phone that connects and gets nowhere, and both clear on a router restart. Resetting the phone's network settings discards every saved network and every VPN profile — try it after the router, not before.
Reset network settings only as a last step
Reset / data riskIt clears every saved Wi-Fi network, every VPN configuration and paired Bluetooth devices across the phone. It is effective, and it is expensive — everything above is cheaper and more specific.
Model-specific
What is true of this model and not of its siblings
Generic advice for the product line fails here. Each item below is a property of this specific model family that changes which of the steps above apply to you.
- Model scope
- SM-S938*. Samsung Galaxy family; S Pen, foldable display, DeX, USB moisture and One UI features vary by exact model.
- Display
- 174.2mm (6.9" full rectangle) / 172.2mm (6.8" rounded corners), 3120 x 1440 (Quad HD+), Dynamic AMOLED 2X, up to 120 Hz. Samsung quotes two figures for screen size — the full rectangle and the rounded-corner measurement — which is why this phone is listed at two different sizes in different places and neither is wrong.
- Refresh rate
- Up to 120 Hz adaptive. A high refresh rate is one of the larger discretionary battery costs on this phone: it is genuinely smoother and it genuinely costs runtime, which is why battery complaints on this model are worth checking against the refresh-rate setting before anything else.
- Panel
- Dynamic AMOLED 2X. This is an AMOLED panel, so dark mode reduces consumption for real — black pixels are switched off rather than backlit — and a static bright element left on screen for very long periods is the one wear risk this panel type has.
- Rear cameras
- 200.0 MP + 50.0 MP + 50.0 MP + 10.0 MP. A 200-megapixel main sensor produces very large files at full resolution, and it is the reason this model fills storage faster than almost anything else in the range. Samsung defaults to a binned lower-resolution output for exactly that reason — a storage complaint here is often a capture-resolution setting.
- Zoom
- Optical Zoom 3x and 5x, Optical quality Zoom 2x and 10x (Enabled by Adaptive Pixel sensor). Zoom range is the practical difference between models in this line. A close-up that will not focus at the long end is normally the telephoto's minimum focus distance rather than a fault — taking the same shot at 1x confirms it in seconds.
- Video
- UHD 8K (7680 x 4320)@30fps. Resolution and frame rate dominate how quickly video fills storage and how hard the phone works thermally — sustained 8K recording is the single most common reason a Galaxy camera stops mid-take and warns about temperature.
- Memory and storage
- 12GB RAM with a 256GB storage tier. RAM matters directly to one symptom: apps reloading when you switch back to them is memory pressure rather than a crash, and it is more likely on the lower-RAM configurations.
- SIM
- SIM 1 + SIM 2 / SIM 1 + eSIM / Dual eSIM. Three combinations are supported, and the default line for calls, for messages and for cellular data are three separate settings — crossing them produces calls arriving on one number while data or messages use another, with almost no indication that anything is wrong.
- Wi-Fi
- 802.11a/b/g/n/ac/ax/be 2.4GHz+5GHz+6GHz, EHT320, MIMO, 4096-QAM. This is Wi-Fi 7 hardware with 6 GHz support, so it can use bands an older phone on the same network cannot — which is worth knowing before concluding the router is at fault when one device performs differently from another.
- Bluetooth
- Bluetooth v5.4. Version affects range, reconnection behaviour and which accessory features are available — a newer accessory paired here operates at this version's capabilities rather than failing, which is regularly mistaken for a pairing fault.
- Connector
- USB Type-C. Note that a USB-C cable which fits is not necessarily rated for this phone's charging speed: USB-C carries several power profiles, and a data-oriented cable charges slowly or intermittently while seating perfectly.
- Samsung's own specification
- Samsung publishes the full specification for this model at samsung.com/uk/smartphones/galaxy-s25-ultra/specs/. Where any figure above matters to a decision, that page is the authority and this one is not.
- Biometric
- An in-display fingerprint reader, which looks through the screen. A thick or incompatible screen protector genuinely interferes with it, which is why unlock reliability often changes the day a protector is fitted rather than gradually.
- Modem
- 5G capable. In marginal 5G coverage the phone hands repeatedly between 5G and LTE, which costs more battery and generates more heat than settling on either — so the preferred-network setting is a real diagnostic lever rather than a preference.
- Charging
- Supports wireless charging. Coil alignment is tighter than it looks and a thick case, a metal plate or a card in the case defeats it entirely — so wireless charging failing while cable charging works is usually the case rather than the phone.
- Water resistance
- Rated water resistant, which is not waterproof and which degrades with age, drops and seal disturbance. The rating covers incidental exposure; it is not a guarantee, and moisture-detection warnings are deliberately conservative and can persist after the port feels dry.
- Platform
- Android, with a manufacturer interface layer over it. Battery optimisation and background-execution restrictions are applied per app and are far more aggressive than on iOS — which is why 'set this app to unrestricted' is a real fix here and has no equivalent elsewhere.
- Symptom boundary
- This record covers Samsung Galaxy S25 Ultra when it will not connect to Wi-Fi or keeps disconnecting. A different symptom on the same hardware, or this symptom on a different Samsung product line, has its own record with its own causes and its own cited source.
Stop boundary
When this stops being a self-service repair
Continuing past any one of these costs more than the repair saves — in safety, in warranty, or in evidence a technician needs.
- !The phone will not see any network anywhere, including networks other devices can see, which points at the radio itself.
- !The router is supplied by the ISP in a locked mode and the setting that needs changing is not exposed.
- !The network is a managed workplace or campus network with device registration requirements only its administrators can satisfy.
- !Wi-Fi fails only after the phone has been dropped or exposed to liquid.
Evidence ledger
Sources behind this answer
Visible sources support the visible claims. Home Product Support records the publisher, the fact used and the review date; community reports can suggest an issue but do not become a published fact on their own.
Where the manufacturer's article stops. Samsung publishes this as "Galaxy phone or tablet Wi-Fi connection issues". Samsung's Wi-Fi troubleshooting article for Galaxy phones and tablets: will not connect, drops the network, or reconnects to the wrong one. Probed 2026-08-21: title-verified. What that article does not carry is the model boundary — it is written for a product line, and SM-S938* is the scope applied here. Where a control label, terminal, indicator pattern or reset sequence differs on your unit, the manual for that exact model is the authority, not this page and not the article.
Exact questions
Common follow-ups
What size is the Galaxy S25 Ultra screen?+
Samsung lists 174.2mm (6.9" full rectangle) / 172.2mm (6.8" rounded corners), at 3120 x 1440 (Quad HD+). The two size figures are the full rectangle and the rounded-corner measurement — both are Samsung's own numbers, which is why this phone appears at two different sizes in different places.
Does the Galaxy S25 Ultra have a 120Hz screen?+
Yes, up to 120 Hz adaptive on a Dynamic AMOLED 2X panel. It is worth knowing for battery complaints specifically: a high refresh rate is one of the larger discretionary costs on this phone, and it is the first setting to check when runtime is disappointing.
Does dark mode save battery on the Galaxy S25 Ultra?+
Yes, genuinely — it uses an Dynamic AMOLED 2X panel, where black pixels are switched off rather than backlit. The saving is real though smaller than reducing brightness or refresh rate, which remain the larger levers.
What cameras does the Galaxy S25 Ultra have?+
200.0 MP + 50.0 MP + 50.0 MP + 10.0 MP, with optical zoom 3x and 5x, optical quality zoom 2x and 10x (enabled by adaptive pixel sensor). The 200-megapixel main sensor is also the reason this model fills storage quickly at full capture resolution — Samsung defaults to a binned output precisely because of that.
How much storage and RAM does the Galaxy S25 Ultra have?+
12GB RAM and a 256GB storage tier. The RAM figure is the one that matters for a specific symptom: apps reloading from scratch when you switch back to them is memory pressure rather than crashing, and it is more likely on lower-RAM configurations.
Does the Galaxy S25 Ultra support eSIM?+
Yes — Samsung lists SIM 1 + SIM 2 / SIM 1 + eSIM / Dual eSIM. Because more than one line can be active, check which line is set as the default for calls, for messages and for cellular data separately; those are three settings and crossing them is easy to do and hard to notice.
What Wi-Fi does the Galaxy S25 Ultra support?+
802.11a/b/g/n/ac/ax/be 2.4GHz+5GHz+6GHz, EHT320, MIMO, 4096-QAM. That is Wi-Fi 7 with 6 GHz, so it can use bands an older phone on the same network cannot — a useful thing to know before blaming the router when two devices behave differently.
Can the Galaxy S25 Ultra record 8K video?+
Yes — UHD 8K (7680 x 4320)@30fps. Bear in mind that resolution and frame rate dominate both how fast storage fills and how hard the phone works thermally, and sustained 8K is the most common reason a Galaxy camera stops mid-recording with a temperature warning.
What Bluetooth version is the Galaxy S25 Ultra?+
Bluetooth v5.4. Version affects range, reconnection and which accessory features are available — a newer accessory paired here runs at this version's capabilities rather than failing, which regularly gets misread as a pairing fault.
How do I tell whether it is the phone or the router?+
Test another device on the same network, in the same place, at the same time. Both struggling makes it the network and every phone-side step is wasted. Only the phone struggling exonerates the router entirely. This one test is worth more than any setting change.
What is the single most effective thing to try?+
Forget the network and rejoin it. A saved entry carries credentials, band preference and captive-portal state, and any of those can go stale after a password change or a router replacement. It costs one password re-entry and it resolves a large share of these.
It says connected but nothing loads.+
That is a useful result: association and authentication both succeeded, so the password, the signal and the band are all fine. What remains is a captive portal that never completed, DNS, a VPN, or the router's own internet connection — none of which are Wi-Fi problems.
Why does it work everywhere except at work?+
Check the private Wi-Fi address setting for that network. Both platforms randomise the hardware address per network by default, and a network that filters by address will reject the randomised one. It is invisible unless you go looking, and it produces exactly this pattern.
It keeps dropping and reconnecting in one spot.+
That is usually band steering — a router advertising 2.4 and 5 GHz under one name moving the phone between them, at the point where each is marginal. Separating the bands temporarily settles it in one test, and the phone's own network detail shows which band it is on.
Should I reset network settings?+
Last, not first. It clears every saved network, every VPN profile and your Bluetooth pairings across the whole phone. Forgetting the single problem network achieves most of the benefit at a fraction of the cost.
The hotel Wi-Fi connects but the sign-in page never appears.+
Forget the network and rejoin so the portal is presented again. A saved captive network reconnects automatically and frequently never offers the page, which leaves you connected with no internet and no obvious way to fix it.
My phone keeps switching to cellular data.+
That is a deliberate feature — both platforms will move to cellular when Wi-Fi is poor rather than leave you with a connection that does not work. If it is unwanted, the setting is a switch. If it is happening constantly, the underlying Wi-Fi signal is the thing worth fixing.
Would a mesh node or extender help?+
Only if the problem is genuinely signal — consistent trouble in one part of the building and none beside the router. Trouble that follows the phone everywhere, or that other devices share, will not improve, and an extender on a congested channel can make throughput worse.
It worked until my router was replaced.+
The phone still holds the old network's saved entry. Forgetting and rejoining is the fix. Setting the new router to the old network name and password is the alternative, and it saves doing this on every device in the house.
Does this answer apply to every Samsung model?+
No. It is scoped to SM-S938*. Samsung Galaxy family; S Pen, foldable display, DeX, USB moisture and One UI features vary by exact model.
What should I record before troubleshooting Wi-Fi or disconnecting?+
Record the complete model identifier, exact message or indicator, software/firmware where visible, the operating stage and what still works.
Why is a factory reset not the first step?+
A reset can erase accounts, networks, maps, schedules or preferences without distinguishing Wi-Fi, disconnecting. The ordered checks preserve that evidence.
When should I stop and use qualified service?+
Do not share Wi-Fi passwords or weaken router security.
Answer for the exact device and software version shown above.