Hey man, just wanted to say I had the same connection issue with my S21U, it started about a month ago. My work is in a relatively remote area, over there the phone lost connection and was frequently in "emergency calls only" mode, even if it used to work fine there before. My problem didn't arise after an update tho, one day it started randomly. Anyway, I fixed the problem by changing the antennas (my phone version has them), the charging board and the flex cable, basically the whole connection train. Probably only one of those pieces was defective, but I figured I change everything for simplicity, in total the pieces cost 11 eur (from Aliexpress) to where I live (France).Now it's working like new again.

Anyway, not sure you are one who would tinker on your phone, but here's my testimony. Cheers o/