Google Pixel 4a (5G) will not connect to Wi-Fi or keeps disconnecting

The checks in more detail

If only one network fails, restart the router, then on the phone open Settings, Network and internet, Internet, tap the gear beside the network and choose Forget, then join again with the password, which clears a stale saved profile. If the phone connects with no internet, check the router's internet light and test another device, and in the network's settings check Private DNS, which can break browsing if set to a provider that is down; set it to Automatic. Signal strength matters on a phone with a small antenna. Move closer to the router and see if the problem disappears.

A router on a crowded 2.4 GHz channel drops devices, so change the channel to 1, 6 or 11, and if the router has a 5 GHz network, join it for speed. Routers with Wi-Fi 6 and WPA3 settings sometimes confuse older phones, so set the router to WPA2/WPA3 mixed mode for a test. Turn off Adaptive connectivity and any VPN, which can interfere. If the phone drops Wi-Fi when the screen is off, check the Battery saver setting, and in Developer options, Wi-Fi settings, since battery optimization can disconnect it. Reset network settings in Settings, System, Reset options, which clears saved networks and Bluetooth pairings, then connect again. Boot into Safe mode to rule out apps.

If Wi-Fi never works on any network after a reset, the phone's radio is failing, which is a risk with a phone of this age, and a repair may not be worth it. When the connection works for browsing but fails for particular apps, consider the router's firewall or parental controls, which may block them, and check whether the app works on mobile data. A quick test is to open a speed-test site on the Wi-Fi network and on mobile data, and compare. If the Wi-Fi result is much lower than mobile data, the network is the cause and the router's settings or position are the first thing to change.