mobile account associated with wrong (old) address

Hi.  I'm trying to help my mother in law get her account straightened out after a move.   She moved a couple months ago, services were transferred to the new address and worked fine.  Last week everything got shut off.  The new address was not visible on her account, and I could not do much on the account, unhelpful errors about 'only the primary account owner can do this'.  Thanks to this forum I found the solution (add the services back as an un-logged in user).  I was able to reactivate her old cable modem and she was good to go.

However, her mobile account is still associated with the old address and we cannot find a way to update that.  The app constantly errors out (probably for the same reasons as above - the other services are off).  She's called and live chatted with agents, nobody seems to be able or willing to help.  

This can't be such a complicated problem.  Can't someone move that service to the new address?  She's at her wits end and has even tried to transfer her service to another carrier but can't because the app throws an error when she tries to get her account number and pin.

Jerry