Sometimes the following tips help but not always.
1. Clear your phone's System Cache Files by turning your phone Off and then from a set series of button presses go into the phone's System Files and clear the cache. No information is lost this way. You may need to insert your headphones or hook upto a computer with your data cable to get to this menu on the phone.
2. Factory reset after backing up and removing any relevant sd card.
It's a conundrum because both my partner and daughter use the Samsung Galaxy s20 5G and have both updated to Android 11 and Samsung One Ui 3.0 and signal and data connection is fine.
Other people have said theirs is fine too so it's going to be difficult to find a common denominator !
I would certainly suggest to give feedback via your Samsung Members App too. If enough people send feedback then Samsung are going to have to sit up and act.
I wish you all the best with this.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.0/ Android 11.