How to activate a new Modem without internet?

I bought a brand new cable modem off amazon. An xfinity approved cable modem. Comcast wont let me activate it though. To activate it through the xfinity website using https://xfinity.com/activate you need an internet connection but naturally of course I dont have that because my cable modem is not activate. I tried bootstraping the process by hooking back up the xfinity gateway but that gets me nowhere. It doesn't let me enter in anything for the new modem. It just brings up options for changing wifi settings in the xfinity gateway. So in other words, comcast is locking me in, forcing me against my will, to rent and use their xfinity gateway.