[Samsung Galaxy Note 10+ Questions & Answers] scottusa2008:
A similar thread was made on the subject - link here https://xdaforums.com/galaxy-note-10+/help/android-10-5g-wifi-dropping-t4037661

For practical intents and purposes it's almost like when the issue happens the phone is not able to resolve a DNS query. I base this on 2 hypothesis - first being that the phone is still connected to the wifi access point and can still access LAN resources. The second being that the phone must either poll the internet (i.e ping a specific address) on a regular basis. If the poll was unsuccessful it flags the connection as "WiFi connected but no internet".

The things you can try are:

update the firmware on your phone (if not already)

factory data reset (read note 1 below)

reset network settings

try a different wireless router (read note 2 and 3 below)

disable private dns (read note 4 below)

If you use tasker create a routine to automate a temporary solution (read note 5)

Notes

The issue maybe a corrupted setting within the phone. I have had similar issues before (not with just this phone) and a factory data reset helped. One thing to take note of is that after the phone had completed it's factory data reset I did not attempt to restore a backup. This is important to keep in mind because any issues that were present (due to corrupted settings) would exist also in the pre-factory data reset backup you made.

With my previous router I had this issue but had since replaced it with a Wifi 6 capable router. After getting the new router I have not had any further issues what so ever. Also make sure your wireless router is up to date on it's firmware. It maybe possible that the wireless router you have is old enough (from when it was released, not from when you bought it) that a replacement might be the only way to resolve the issue.

To help rule out a wireless router issue try using a different one. You can borrow a friend's internet connection for a while, or a public cafe or what ever. Point is you are trying to see if the issue happens again when you are using a totally different wireless access point (by totally different I mean hardware/model and not just renaming the SSID).

I had the issue happen when using private DNS. For what ever reason the issue was with the DNS provider I had choosen. Turning the Private DNS setting off helped resolve the issue. After turning the setting off it may take a bit before the phone recognizes it was turned off, but you can hasten this process by toggling WiFi off and back on.

The Tasker option is one I did and it worked quite well. You may need to adjust the routine to suit your particular needs but it's basically what is at this link here. Best part of this option is that you can have it only activate when your connected to the problematic wifi access point and every X number of minutes (helps ensure that you use it only when needed). The routine (when setup correctly) should run every 10 minutes in where it attempts to access a website and assigns the results to a variable. The variable is then checked to see if it matches a specific result (in this case an error code), and if it does match it will toggle WiFi off and back on (after a brief pause/wait period between toggle off and toggle on) automatically. Granted it sounds like a lot of work, but it's really not that complex. Also I consider this is a temporary solution because you may find that on later firmware updates applied to the phone (or you may end up getting a new wireless router) the issue will have been resolved.