Look at Billing on line (https://www.xfinity.com/mobile/my-account/billing) which requires your login.
There you will find 2 links under Payment Method (CArd details which then displays the Card KInd and the last 4 digits, next to it is Update Card which allows you to change evrything about the card.)
Be sure to log out, close your browser and the be certain all the details are correct. Note: There is a different process for your cable bill.
The instructions are based on the desktop version, both are similar.
Be sure to SAVE your entry, by clicking on the save button
Glad you found a solution, I have never had a broblem with the desktop version.
Did you update the mobile site or the cable one you have to do both!
The numbers you call are seperate for mobile and cable