Stop paying for modem rental - been renting for 2 years

Hi,

When I started purchasing WiFi from Xfinity I was told that I could buy out the modem after I had rented it for a long time. I have been renting it for 2 years, and I want to buy it out. I preferably no longer want to be charged any additional fees because at this point I have paid more than the modem is worth in rental fees. The rental fee has even increased over the past 2 years and I no longer want to pay it.