Comcast ARRIS and Back to My Mac?

I have the Comcast ARRIS modem/router and an Airport Extreme (5th Generation). After realizing the modem/router was the source of my Guest Network failing, I had Comcast completely disable the router/wireless feature (not just put it in bridge mode, but disable the signal altogether) - which after more reading today, I think may end up being a regular call - sounds like the wireless feature resets to "on" with any modem reset or power outage. Anyway, the Guest Network now works great again. However, Back to My Mac (which worked previously) has not worked since I got the new modem and still doesn't work. All of my settings on my Mac and Airport Extreme are correct. I'm wondering if there's something through the admin function on the modem that I have to change as well? Could a firewall setting be preventing Back to My Mac from working? By the way, I can access the Mac from my MacBook when I'm at home on the same network with no problems. Anyone else have this issue with Back to My Mac? Thanks!