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.
They are phasing it out but I'm going to escalate your issue to the Comcast corporate employees that are available to these boards. We've seen cases here where they have been able to get this accomplished for customers even when it is not listed in their rate cards, YMMV. You should get a reply here in your topic. Good luck !
Canceling the escalation. Good luck ! Thread closed.