Network issues (DNS?)

I having weird network issues internet connection seems to break down very short periods (1-2s) really often (couple times for minute). Problem is same on wired and wireless connection and some apps work without problem but browsing causes constant 'Your connection was interrupted - A network change detected'. I think it could be related lates Big Sur updates.

I noticed also that some reason after boot up there is 20-30min clear period without any problems.

Also there is problem that I can not change DNS provider on network settings as it keeps unselecting/canceling my changes (on about same period with network brakes).

Tried already: flushing dns cache, restarting mDNSReponder, restart laptop and running latest updates without any noticeable effect.