Being charged rental fee for my own modem

I have been paying a $14/mo rental fee for a modem that I own since the beginning of the year. I purchased the modem 8 or 9 years ago from Fry's Electronics. I have been using it for my Comcast internet service since that time and across several service addresses. I changed my service to a new address a year ago and my modem was reclassified to a rental. I have spent hours on the phone this year with Xfinity customer service agents who promised they were fixing the issue, and yet I am still being charged $14 every month. I have been given refunds, yet no one has been able to remove this from my bill. It is beyond comical how impossible this task seems to be. The whole of Xfinity customer service cannot figure out how to stop charging me for my own hardware. I started calling about the issue last December, so we're coming up on one year of Xfinity wasting my time on this.