iPhone12 pro max bought from Apple....Cannot be used on xfinity??

Needless to say I am very angry to find out that you can only BYOD if you have an iphone 12 pro MAX that you bring to Xfinity from ATT,Sprint,Verizion and Tmobile they can change your service.

But not from a phone I already purchased from Apple . However I can take my iphone I bought from Apple and activate it on ATT,Sprint,Verizion and Tmobile.

They said there is NO eta on when I could swap my current iPhone to the new one.

So frustrating. Guess I will have to take my new iPhone to a carrier that can support it.