First, check the "Rate Card" for your local area to see if that feature is even available anymore. Seems that it is being phased out;
It's a PDF.
Scroll down to "Xfinity Internet" and look for "Additional IP Addresses".
If nothing is listed then it's likely to have been discontinued in your local market area.
If you're teaming then multiple ports show as a single virtual port and only get 1 IP.
I have my setup with a MB8600 and 2 cables from there to my machine with LACP configured... Depending on how you have things setup you might have to add a delay statement of 3-5 seconds to prevent the system from not getting the same IP and requiring a reboot of the modem to get a new IP for the alternate MAC the modem is seeing as up first.
iface enp2s0f0 inet manual
iface enp2s0f1 inet manual
pre-up sleep 5 ##causes a 5 second delay at boot for the CM/MAC binding to be consistent