So if I am reading your post correctly, you are plugging your home router into port 1 and the XH router into port 2 of the Arris TG862? The Xfinity Telephone/Internet Modem I have is the Ubee dvm3203b which has just a single ethernet port and a single RJ-11 phone port.
While my current config of connecting the XH router to my Netgear home router (which in turn is connected to a Apple AirPort Extreme which connects to the Ubee modem) eliminated the degraded service issue I (and you) had, my irritating problem I get is the XH router occassionally assigns IP addresses (172.) to our mobile devices. It sounds like the Arris modem would solve my issue, and I believe this multiport modem solution has been suggested in previous posts.
I'm going to put the "Call Comcast and request Arris TG862 modem" on my reminders list! Thanks for sharing!