Samsung Galaxy S24+ has no cellular service or data

The checks in more detail

and a phone set to the wrong line for data or calls looks like a dead network. Check Mobile data is on, that Data roaming is on if you are abroad, and that Airplane mode is off. Toggle Airplane mode for ten seconds to force a new registration. Next look at the SIM. Remove the tray with the pin, clean the SIM and tray, refit it and test, or try the SIM in another phone, and try another SIM in this one to see whether the phone or the SIM is at fault.

If you use an eSIM, remove and re-add the plan from the carrier. In Settings, Connections, Mobile networks, check Network mode: setting it to 5G/4G/3G/2G auto is normal, and trying 4G only can fix a phone that struggles with a weak 5G signal. Reset network settings in Settings, General management, Reset, to clear carrier profiles, then restart. Check the carrier's status page for outages and ask whether your account or the device is blocked. Install software updates, since carrier settings arrive with them. Test in Safe mode to rule out apps.

If no SIM works in this phone, the antenna or modem hardware needs Samsung service, and you should back up first. Samsung sells the S24+ with different modems by region, so settings and supported 5G bands vary; check the carrier's list of supported bands for your model code, found under Settings, About phone, if a particular band is poor. Also look at Wi-Fi calling in the Phone app settings: it can fix indoor coverage when the cellular signal is weak, and if it is on with a poor Wi-Fi link it can cause dropped calls, so test with it on and off.