Samsung Galaxy S21 will not connect to Wi-Fi or keeps disconnecting

The checks in more detail

If the router merges bands or uses Wi-Fi 6 features such as OFDMA in a way the phone dislikes, update the router firmware. Check that Private DNS under Settings, Connections, More connection settings is not set to a server that fails. A restart and a cache partition wipe help after the last updates. Because the S21 is old, a weak antenna or a bent case can reduce range, so test without the case. Use Samsung Members diagnostics to test the Wi-Fi radio, and reset network settings if everything else fails.

The S21 has a plastic back on the standard model and a metal frame, and its antenna design makes it sensitive to how the phone is held, so test with the phone resting on a table next to the router for a clean result. If speeds are fine next to the router but poor at your seat, the router's coverage is the problem. On a router with Wi-Fi 6 features such as OFDMA and target wake time, older firmware can have bugs with Samsung phones of this age,

and turning those features off in the router's settings, if offered, stabilized some networks. Change one setting at a time, and check the router's release notes for fixes related to Android devices. It is also worth checking your phone's date and time, which should be set automatically: a wrong clock stops a phone from validating secure connections and can make Wi-Fi appear connected with no internet. Settings, General management, Date and time should show Automatic date and time on, and the time zone should match your location.