New Apple watch is stuck on "activating". I've purchased data add on for the watch through my o2 account. I have reset/re-started both the watch and the phone multiple times. The problem doesn't seem to be the watch (which works in every other respect) but with o2.
It's worth disabling 5g on your phones as some have found that fixes it whilst O2 sort out the issue.
You need to reboot phone and watch after.