I had to wipe my iPhone x and restore it from backup. In order to pair my Apple watch 3 back to the phone I also had to reset the watch (note that I had cellular data plan for this watch). I was able to get the watch to pair to the phone again, but the cellular data plan will not load. When I go in the watch app on my phone, it says set up cellular. I click on it and it spins for a long time but nothing happens- keeps telling me to finish setting up my watch and to set up cellular.
I have powered both the phone and watch off and on but that didn't help.