@Nigel-tufnel I'm on the same hardware and indirectly using AT&T (an mvno derivative). For the most part, things have been working well for me...data, text, and calls.
Not the expert here, but have you confirmed it's actually unlocked? ie: get it to the bootloader and run fastboot oem unlock from your machine?
I'm on OTA-16 here and it worked on the build version (from Nov/Dec '20??) before as well.