Returned Router On Autopay Charged Many months

We moved 4 plus years ago, set up our new router and returned the old at the drop off spot. Received email verification of the return. We recently updated our account asking for paper bills as I have a small business and like to have a paper copy. After this update and a closer review of our monthly bill, we realized that we were charged $15 a month for the past several years for a returned router. We feel that this over sight of $800 plus dollars is non forgivable. We have called several times and escalated with only a closed case , no resolution. Basically a ticket was opened and closed with no resolution. When we call we get an off shore agent repeatedly and no one seems to be able to help. We feel it’s extremely unfair to demand payment for services and when there is a problem Xfinity avoids the issue all together. They offered us a $15 credit for 1 year in which we refused due to the principle and were told to call back after 30 days to dispute in who we did. We still have no resolution. Over 5 hours spent with agents just last week and no one seems to know how to help us with a credit or refund.