Removing personally-owned modem/router from my Xfinity account?

I would like to remove my personally-owned modem/router from my Xfinity account so that I can sell it. (I have just received an XB8 to replace it.) How do I go about doing this? I’ve seen answers to similar questions indicating that there is an option to do so in the Xfinity app, but I don’t see it there; other answers suggested sending Xfinity Support a direct message through the Xfinity Community forums, but I don’t see the direct message option, either.

Thanks very much!