I have an Apple Watch 3 that I previously used on and was purchased from Verizon that I am attempting to connect to my unlocked iPhone 11. I have received the "Uh Oh" message after getting to step 4. I have also gone to the Cellular area and saw that "Xfinity Mobile" showed up for a brief second then suddenly disappeared as I have seen has happened with previous customers. I have gone through the Xfinity forums, Apple forums, spoken to several Xfinity customer service agents, gone to the Xfinity store (only to be told that they "don't handle that") and done all troubleshooting. I have tried resetting the network settings, turning my phone off and on, disconnecting and connecting my watch several times, deleting and downloading both the Xfinity and Apple Watch apps, attempted to connect with the WiFi and without WiFi, checked my imei to confirm that the device does work on Xfinity... I've done it all.
There are about 6 or 7 topics about this on the forum now, I am guessing this is an Xfinity server issue at this point. The odds of having this many people with the same issue at nearly the same times seems too coincidental to me.