Modem Rental Charge on Bill

Hi, I am being repeatedly charged for a modem rental fee, when I do not use an xfinity supplied modem. I chatted with a customer service agent previously about this, to whom i provided proof that I own my modem. He then said he was taking the charge off the bill, but I have still been charged for it each month. As a result, I have a balance in arrears, although I have been paying the correct amount ($70) each month. The transcript of my previous customer service chat will show that this issue was already supposedly resolved. How can I get this charge removed?