I've got the same / a similar issue. Do you find things work if you've only got one SIM in the phone?
My issue seems to occur only when two SIMs are put in the phone, and it only affects 1 of the 2 SIMs. I've just added a thread detailing my issues and steps I've taken to try and fix it, including a brand new S21 which also has the issue.
And thats bull****, its issues with The Phone, got The same ***** where The network just stops working and I live in Sweden.