For the modem, purchase one that follows their specs.
You can find that right here:
XFINITY Internet Approved Devices
I bought mine on Amazon. It has been awhile but I think it was around $70.
Once you get the modem, you can hook it up in place of theirs and provide them with the serial number.
Make sure you then return theirs or you will continue to be charged for theirs.
Also watch your bill to make sure they list the one you purchased as a customer owned modem and you are not charged.