I purchased a new Motorola SB6120 about 6 years ago from Best Buy and used it with Comcast. They started billing me for a modem rental back then and I called and it was corrected. I moved several times and switched providers and I haven't used this modem for several years. I kind of forgot I had it until I did some cleaning of my old storage locker and rediscovered it. No reason for me to be paying Comcast who I am using again for a modem when I have one so I tried to activate it today and Comcast is again claiming it is a rental. I see this is a pretty common problem. I also now see my modem is now listed as a rental on my account after I used customer support to activate it and went up two levels to get it taken off as a rental. Customer service tried their best but my modem is showing as a rental on my xfinity page. I am returning the actual comcast unit tomorrow but I need this straightened out and comcast to recognize this is my modem so I do not get charged for it.
Unfortunately, this happens a lot. It's happened to me, as well, and as it was explained to me, the modem ownership status "switch" needs to be flipped in two places by two different departments, which often doesn't happen. Then, when an automated audit happens, the AI thinks the modem belongs to Comcast and generates the letter. A Comcast employee will need to address this; they answer problems in the order they are posted, so it may take a while.
The SB6120 is obsolete. It's reached end of life status. I'll ask an employee to take a look anyhow so you don't have a billing issue.
I've asked a Comcast employee to help you. You should expect a reply in this thread.
Sorry for the delayed reply. I can look into this for you. Please send me a private message by clicking on my name, "ComcastChe" then click private message me. In the body of this message please verify your first and last name, street address including city state and zip code, and the account number or phone number associated with your services.