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.
Hey there, rowleystreet, welcome to our community! I just got your PM and responded there to see if the additional IP is something available in your area. Looking forward to hearing back from you there.