Xfinity Mobile REFUSES to change my IMEI#

After trying to upgrade to the new iphone 11 pro max, I was informed that my IMEI #s did not match and they would handle the situation in the store. I said okay and waited 2 hrs for this to end up NOT being resolved and I would have to wait. 

It has now been two weeks and countless phone calls for them to match my IMEI#s and Ive had it. They keep telling me that they cant reach their tier 2 contacts to see what the issue is with upadating my IMEI #. I dont understand why they cant fix my problem as in all they have to do is change one sequenece of numbers to match another. PLEASE HELP