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

The checks in more detail

Check Private DNS under More connection settings and the Wi-Fi diagnostic in Samsung Members. If Wi-Fi turns off after sleep, check Adaptive battery and Put unused apps to sleep. A weak signal at your seat is a coverage problem for the router, not the phone. A case with metal can reduce range. A cache partition wipe after the final update clears leftovers, and a network reset rebuilds the stack. The S21+ is old, so a radio fault after years is possible, and the diagnostic says so.

The S21+'s glass back and aluminum frame form an antenna structure, so a thick case with a metal insert or a magnetic mount can detune it, and moving to a thin case or no case often changes signal bars noticeably. If your Wi-Fi is fine at the router but not in the next room, the issue is router placement and not the phone, and a mesh node helps. It can help to forget and re-add the router's 2.4 GHz and 5 GHz networks one at a time, since a phone that has stored both under one name sometimes holds the weaker one.

Another cause on long-lived phones is a corrupted Wi-Fi configuration file, which a network reset rebuilds, and after the reset the phone usually behaves like new until a router change breaks it again. Look at the date of the router's last firmware update: a router that has not been updated for years can hold bugs with newer client chips, and a phone of the S21 generation sits in the middle where both old and new behavior show. Updating the router is often the one change that fixes a phone's Wi-Fi without touching the phone at all.