You can flash t-mobile firmware as long as the build number is the same as ATTs current build. Once you put your sim in youll be on ATTs firmware. I had this issue awhile back.
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Guess what! I'm here with the same issue. S21 ultra and galaxy watch 4. ATT reps don't have a clue on what to do or how to fix it. I've been seeing online that this is becoming a pretty big problem so it should get fixed soon.
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