This question seems to be addressed in the forum. But the solution was discussed offline. So I still can not figure out, how to do it?
Can one of the xfinity experts help ?
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.