Warning for all...
I signed up for Xfinity mobile about 6 months ago and brought 4 iPhones with me when I signed up for service. One of those phones broke and we replaced it with a phone directly from Apple, but now it cannot connect to any of the Xfinity Hotspots. Customer service told me that their system will not allow them to replace one BYOD with another BYOD on the same line - I went to an Xfinity Store and called customer service and got the same answer from both. The phone works, with the exception of not being able to access the hotspots, which actually surprise them. I told them it was a serious gap in their customer service model, as I would imagine this will affect many customers (and for that fact, some of their own employees) in the future. The workaround to the problem - add the new phone as a new line and get a new phone number for it.
Caveat to this whole thing - If you purchase a phone from Xfinity to replace your BYOD, that can be done and it will work correctly.
Below is a link on information on Xfinity hot spots if that is your only problem?
I just discovered exactly the same thing -- how utterly absurd that an existing BYOD cannot be replaced with another. In my case, I initially just moved the SIM from the old phone to the replacement, and everything worked fine. However, I noticed that the Xfinity Mobile site and app showed the wrong details about the phone, so I contacted XM chat support to fix it. 4 chats and two phone escalations later, I was finally informed of this nonsense. The final tech (who sounded like they knew what they were doing) said the best he could do was update the IMEI / ICCID on his backend system -- which would allow the phone to switch between By the Gig and Unlimited normally (I've never done that but want the option). But to make the phone look "right" on the web site "front end" would require starting with it as a brand new BYOD from scratch. I didn't get the impression that meant I couldn't keep the phone's existing number, which would be even *more* ludicrous, but maybe I just didn't catch that.
Bottom line -- Comcast, this policy is bonehead. Fix it. People have to replace or upgrade their phones all the time for various reasons.