I did try searching for my situation but no luck so here goes:
I have one of the recent Advanced Wi Fi modems from rogers (I forget the model #).
I also have a DLink router.
Using all of the same cables and equipment, if I plug my PC and my Xbox into my Dlink router, my Xbox is able to see my pc and can play videos from it.
If I take those same cables and move them over to the Rogers modem, the Xbox cannot see the PC at all.
So there's obviously a setting on the Rogers modem that I need to change, but I can't seem to figure out which one it is. I tried calling Rogers but I'm not entirely sure they understood what I was trying to do, and the tech's solution seemed to have me doing nothing except unplugging and plugging in the cable modem over and over.
I should mention that when the xbox and PC are connected to the Rogers modem, everything else works perfectly. The only thing I can't get it to do, is let the Xbox and PC talk to each other. I have verified that they are both on the same IP range, same subnet etc...
What IP are your computer and xbox assigned when plugged directly into the Rogers modem? this will give us an idea if your being assigned a private or public address on them and thus deduce if your modem is in Bridge or Gateway mode. Do they start with 192.168.X.X or something else?