My wife’s OnePlus 8T was upgraded recently to OxygenOS 12 and she lost carrier access. We tried everything we could find after Googling the issue, but nothing worked.
I looked up the Access point Name settings and we noticed that it was set for ATT Nxtgenphone, even though Cricket was an option. Setting to Cricket didn’t help, until I noticed that the APN read “ndo”, not “Endo”. I fixed that spelling error. All the other settings for the Access point were correct.
Within a few minutes, her cellular service started working again.
Just throwing this out in case it helps someone else.