You're not alone, six of the top 10 topics or so are about this issue.
I have tried atleast 20 times in the last few days since getting my new watch from xfinity.
I have spoke to 3 different chat agents (useless in my opinion)
1 phone agent (helped best she could but only ended up making ticket)
1 Reddit agent (did what they could, but hasnt responded in a day)
and 1 forum agent @ComcastChrisL (actually deleted the old failed line from plan, although didn't do the trick)
So far all have atleast been polite and tried, but still nothing. I have a ticket open and haven't heard anything in 3 days since openning the ticket.
I have tried on WiFi, on Cellular, resetting the phone and watch, factory resetting the watch, factory resetting the phone (mad I did this, major waste of my time). I have let it sit on the 4/4 page for an hour plus a couple of times. I did it right before going to bed and didn't touch it all night.
I have a series 1, the main reason I upgraded to series 5 was the cellular use. The other updates are nice, but not worth the price without the cellular feature.
This seems to be a continuous issue, really hoping there is a blanket fix soon, as I am getting quite anxious about it. I am pretty sure there are no other things I can try to solve this. 😕
It's sad that this can't be simple to fix or do. I spent an hour or more on the phone with a rep but nothing helped I did all the things she wanted me to do prior to calling.
This is frustrating the watch works as long as it's close to the phone to call and receive calls and text on the cellular only.
I bought this as a gift for my wife. Must be something going on with Xfinity's system.
There's so many people with the same issue.