really really odd problem you have there.
ok so its set up like this SACM to router then to pc via ethernet wired i assume, and then to the XBOX - wirelessly ??..thats a bunch of cable if not.
there are several revisions of the router (i have one myself) which can be determined from the serial code on the underside, google it or
www.linksysnfo.org (crackin wee site). From there u can determine which of the current firmwares is relevant.
In the settings is it set to act as a Gateway or a Router, gateway is the answer.
This is a great piece of kit i promise you, i've updated the firmware numerous times, run an Xbox wirelessly, my wife's laptop wirelessly, get full download speeds, have been intrusion free and plan to incorporate it into an AV set up with itunes in due course. Keep posting will try to help.