Spectrum is unable to activate cellular service on my new Apple Watch Ultra 3

Ditto with my situation Bought mine direct from Apple. Spectrum is not allowing Ultra3 BYOD. So, if you don’t buy the Ultra3 from Spectrum, they won’t allow it to be added at this time.

To me it sounds like you just got a lazy or incompetent customer service rep. I am on spectrum mobile, as are my wife, son and daughter, we all had a variant of the iPhone 16 and all had watches through spectrum, my son and daughter each had a series 8, my wife a series 9, and myself an ultra 2. We all upgraded, I got a 17 pro max and Apple Watch Ultra 3, my wife and children all got iPhone Airs and series 11 watches. Ordered and upgraded online, when they were delivered, switching everything over to the new devices was absolutely seamless. Never had any type of issue getting the new watches to activate on our accounts in place of the old. I would go in to your local spectrum mobile location, or try calling back and hope you get someone more willing and able to help you.

Did you get your ultra three from spectrum or did you order it from Apple. I got mine from Apple and they consider it a bring your own device instead of purchased through spectrum. They would not activate my ultra three as a BYOD let me know and I will try it again if you bought it direct from Apple.

Did they consider to set up a new plan for you, including new phone number and eSim for the watch?