Desperate for help. Being charged for my own modem

Hoping someone out there can help me get to the right place. Here's my horror story.

In 2009 I went to Best buy and purchased a Motorola surfboard modem SB6120 and used it at my old home until 2016.

In  2018 I got married and we bought a house together and reestablished a new account with Xfinity. Since the new home we bought previously had Comcast connectivity previously all I had to do for internet was a self-install using again my own surfboard modem. The way Comcast has been billing me for the past 5 or so years was they had a $15 rental with a negative $15 credit to zero out the equipment since it was a customer owned modem. 

In May I got a notice that they were changing up their billing system and that they noticed they had been doing my billing incorrectly.

That's when the trouble started. Since May I've been being charged a $15 equipment / rental fee on every bill. I've spent hours on the phone talking in circles. I've explained over and over that the surfboard modem I have with the MAC address that they see on my account as my own personal device. But all they will say is we have looked into the matter and this MAC address is one of our devices. I've told them also that I have the original pink carbon copy slip from Comcast in 2009 that clearly states customer-owned modem and has my MAC address printed on it. They say there's no way for me to send that a picture of that to them. On my third call with support they suggested that I needed to go to the store physically and bring my modem to prove that it was not a piece of Xfinity equipment. I took a Saturday and went to the store and explained my issue and what I was told to do by the phone support. The associates at the store looked at me like I was crazy and said that the surfboard modem was definitely not anything they lease. They were even skeptical but when they scan the barcode with the MAC address it pulls up my account in their system. They said they didn't know what to do and that I would need to call the 800 number as they did not have a direct number to any type of help. I stayed in the store while I waited on the phone for about 45 minutes and the person that I spoke to seemed to understand the issue and said that they had resolved it and gave me a credit. The next month I'm again being charged the $15 for my own modem. 

Does anyone know anyone that can possibly help me get this resolved? Phone support says they open tickets and I never get any phone calls back. When I call back myself and ask for a status I'm told they looked into the matter and that the MAC address matches one in there database. Don't feel like any deep research is really being done on this like checking that I don't even have an equipment agreement with Comcast. At this point I'm willing to give them the worthless modem and just buy myself a new more modern one. But to add to the craziness if I go to the my equipment section when logged into my account it says there is no equipment leased by me so therefore there's no way for me to return it or get a return label.

My internet options in my town are limited so Xfinity is my only true choice.