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 !
how do i get it?
hey there! Thanks for posting about getting a second IP address on your modem. Appreciate you using the forums. Glad to have you here! Only a few regions of the country have access to this feature. In my experience, it's usually in places in the western US. I've done a couple before in Cali accounts.
Let's check to see if this is an option on your account. If so, it's like 4.99 or so a month for the additional IP. Let's get started.
Please send me a private message with your full name so I can take a look at your options (click my name Comcast_Support, and on the next page is an option to send a private message on the right).