Samsung Galaxy Note20 will not connect to Wi-Fi or keeps disconnecting

The checks in more detail

Check Private DNS and the Wi-Fi diagnostic in Samsung Members. If the problem began after the final update, wipe the cache partition. The Note20's case, particularly one with metal or a magnetic ring, can weaken the signal. Test the phone on a hotspot to see whether the phone or the router is at fault. A network reset rebuilds the stack. A radio that has failed with age is possible; the diagnostic says so, and a repair or replacement is the choice for a phone this old.

Because the Note20 is a large phone, its antennas sit around the frame and the S Pen slot sits at the bottom left, so holding the phone in a way that covers the top edge or the left side with your hand can reduce signal; try the phone on a stand to compare. If your router offers a separate IoT or 2.4 GHz-only network, avoid joining it with the Note20, because the 5 GHz band gives better throughput. As the phone is old, its Wi-Fi chip may also be limited to older security modes on some routers:

if your router is set to WPA3-only, a Note20 may refuse to connect, and setting the router to WPA2/WPA3 mixed mode lets it join. Check the router's security setting if the phone sees the network but cannot join with the right password. Another detail on an older phone is the saved Wi-Fi network's IP settings: if a network was once set to Static, the phone keeps that setting and may fail on the new router. In the network's details, change IP settings to DHCP and the phone gets an address automatically, which fixes a connection that says Connected without internet after a router change.