Third-party router still linked to Comcast despite me no longer using it

Hi there! I have a third-party router and modem (purchased myself from Best Buy, not Xfinity equipment) that I used to use for my Xfinity account (I now use an Xfinity router and haven't used the third party items in years). I gave away the third party router/modem to a friend, who is unable to use it because Xfinity still thinks it is associated with my account. I can't find it anywhere in my account settings, and have been unable to get a hold of an actual human on the phone to resolve this. What can I do to unlink this router from my acct so that my friend can use it?

Thank you so much!