Charged rental fee for Purchased Modem

I've been getting charged a rental fee on a modem I bought directly from Comcast and that isn't even operational anymore. I've contacted support 3 times now and each time I hear the same thing that they are going to do an investigation and get back to me within 48 hours. Of course no one ever does and another month clicks by with another $14 charge. 

Here's what happened. I rented a voice modem for years. Back in 2017, I finally decided to buy one of the recommended modems at Best Buy. After almost six hours of various calls with support they were unable to initialize the modem from Best Buy. They did however offer to sell me the voice modem I had been renting for $200. I agreed, was charged the $200 and the rental charges ceased. The charge for the modem showed up on the billing statement. 

Fast forward to August of 2019 when I did a rate and service change on my account. The rep completely botched the process and the whole thing had to be redone. Unbeknownst to me, the rental charge for the modem was reinitiated at that time. 

My problem now is that I can't go far back enough to get the statement that shows the modem purchase. What can I do on this? With all the rental charges over the years, I've paid close to $1000 for this voice modem that I no longer use of even have.