My 21 Ultra is biggest hunk of junk I've purchased from Samsung the last 10 years or so. It has so many issues that I'm completely unsatisfied with this phone. Issues range from strength of signal (so many places my S8, S9, S10, and S20 had service where as this one will have the no service symbol [:prohibited:], to freezing up the text message app (if it doesn't send a message to somebody because the phone can't hook to a registered network and it ask if you want to resend... it's a waste of time and it won't send a text to anyone else either after that. I must turn phone off and back on to text anyone again.), and the keyboard is screwed up to ( when I hit the symbols key it changes to symbols for about .5 (half a second) then reverts right back to letters. Impossible to to get a symbol typed out that quick. There are others but that is enogh for now.
Wish I had my 20 back.
Is your S21 still under warranty? Send it back and ask for a refund. Call around to, like best buy. I'm sure there are plenty of S20 Ultra's around if you want to go back to it. Your phone seems definitely defective.
regarding messages issue, i would contact your carrier and see if it's like a temporary network issue.
i think you should submit an error report via samsung members app for other software issues (members app > + button > error report)