I have been struggling with sending text messages on my Samsung S21, as well as issues logging on to the ID mobile app.
Tried all the advice on here and nothing worked.
Then realised that on setting my Samsung to power saving to extend battery life, the phone dropped to 2G/3G, not 5G.
Turned off power saving, returned to 5G and the issue vanished.
I wanted to make a post here in case it is of any help to anyone else driving themselves batty!