You've made some progress but let xfinitry support configure it the first time to work with your system.
I just signed up for xfinity internet (no cable/TV) and I've got this modem, an Arris TM602G/CT that I found in the dumpster at my apartment complex -- someone was moving and threw it out along with a few monitors and desktop towers. It still works, I can connect to it with an ethernet cable. A neighbor has an xfinity access point nearby but the signal is sketchy, though I'm able to login with my own account and use it. How would I go about setting up my own xfinity access point using this modem? Do I need a special modem and/or router?