Esim is working on my locked T-Mobile S21 ultra. Did not use convert to esim but activated on the T-Mobile website with the EID. Took about 5 minutes to get a message that it was activated as of be patient.
Same for me as well. Unlocked Tmobile here. Would work on wifi calling, but never connected to cell. Error message when searching for networks as well.
I got mine to work through TMobile using sim restore option. Went through the process of logging in to TMobile site and the phone did the rest. I have esim TMobile and ATT sim working perfectly in the US. I'm using the unlocked 988U1 s21 ultra.