Series 7 Apple Watch not connecting to Xfinity mobile cellular

have you updated the software on both the phone and the watch??

You'll need the form on this topic that was closed for some reason: Apple Watch Activation issue | Xfinity Community Forum

Fill out the mentioned form and someone will contact you.
They will begin a process to add your watch to the valid inventory (whatever that means).  This allows it to be activated.
Get the persons contact information in case you need to call them back. 
After several days, and after they get a notice that the watch has been added, they will activate it and call you.
You will also get an email that says "Activation Complete".
You then, on your watch app click to add cellular, and after several seconds Xfinity will show up as your carrier.

Anything else you try will be a waste of time - don't call Apple, don't go to the Xfinity store, don't reset and re-pair your watch.  None of that helps.

Even with the form, I was unable to get my watch activated.

Hello! Thank you for taking the time to reach out to the Digital Care team! We have a dedicated team of Xfinity Mobile experts that can assist you with your issue, and this team can be reached directly via one of the following methods:

SMS Text Message: 1 (888) 936-4968
Phone: 1 (888) 936-4968
Chat: https://comca.st/3EFXruU

Thank you! -Ty

I having the same exact problem and it’s very frustrating

Good luck, the Apple Watch activation experience with Xfinity Mobile has been frustrating. I opened a ticket due to activation issues with an Apple Watch series 7 a month ago as the issue had to be escalated to the highest level support. I contacted the Executive Resolution link from the other thread and got a we understand that is frustrating but we don’t really over see Xfinity Mobile but we are aware of a network level issue with activating series 7 Apple Watches. After one of the attempted activations an Apple Watch line showed up  on my account charging me $10 a month which got credited but still cost me at least 2 months of the 3 free months you get for activating an Apple Watch. To get the Apple Watch line, that never actually worked or was activated, cancelled I had to chat or talk with 5 different people over a 2 week period. I’m debating at this point whether to switch cell providers to use this Apple Watch or sell my Apple Watch and get a non-cellular model. 

I call every 2-3 days and have tried activation well over 100 times to new avail.  Apple even tried sending me a new watch.  

Basically Xfinity is broke and last two calls led them to tell me.  We have no idea if it will be fixed.  Just keep trying like when you press the elevator door close button.

Mine took 28 days to activate.  Eventually it was manually added, and I did not activate it myself.  It just worked. 

I have been having the same issue . Xfinity says the watch has been deactivated. They say it can’t be reactivated. Which makes no sense since it was bought from them only a moth ago . Asked who deactivated it and why and they say the don’t know. 

I too am having this same issue. If this is a known issue why isn’t it communicated to customers?!? The 10 representatives I’ve spoken to in the last 2 days acted like it wasn’t a known problem, and obviously it is! I would have never ordered the cellular version if I knew this. Has anyone had luck on the cellular activation or did everyone just give up? 

I didn't give up; I did what I posted above and have been enjoying my watch for 6 months now.