My S10 with Android 11 worked fine with Samsung RCS, got an S21 last Monday and have been trying to sort it ever since. I've also tried Google Messaging and researched it like crazy but have found no solution. My learned friend @ironass has it working but my device fails to register and we are both with the same carrier. I had thought of sending back the S21 Ultra but after disabling RCS in the S10, I couldn't get connected. These problems are widespread, wish I could help.
this is strange because when i got s21 ultra i had chat working then it stopped but after a few weeks spoke to samsung and they said do a soft restart and as soon as i did that chat started to work.. woke this morning and noticed chat toggle is missing and it's also missing on my wife s9 we both are with virgin mobile done all that could be done to try and get it back again and still no luck..