Xfinity Problem with Owned Cable Modem Box

So I've owned my cable modem box for YEARS (Xfinity brand) and about a month ago, my Internet stopped working. They were unable to help me over the phone because they couldn't enter the serial number to get it online again, and told me to go into an Xfinity store. I went into the store and the manager there said the Xfinity had done a software change or something and wiped out the serial numbers of all people with the type of Xfinity modem I have. She was also unable to enter my serial number manually but gave me a loaner box and said that a technician would call me once the problem was fixed in the Xfinity database and they could enter my serial number to get my cable modem back online. Well it's been almost a month and I have tried to call customer service and no one is aware of this problem and no technician has called me back yet. I feel like I have no option but make another appointment and go back into the store just to ask if the problem has been taken care of unless someone here knows the answer. Please advise!! I don't want to have to call Xfinity customer service every month to ask to waive the $14 rental fee while my problem is being addressed (assuming it is still being worked on?!)