Xfinity Mobile

I am trying to get a device with Xfinity mobile but every time I want to pay it says I have to update my automatic billing method. When I try to add a different card it doesn’t let me or says I don’t have Xfinity mobile which is true, but I had gotten a device that I ended up returning and not keeping and ever since if acts like I have móbile / won’t let me add a new card or take off the “old” one. Help! I’ve tried chat and nobody ever helps.