I have the same problem... Came here hoping someone found an answer for this router. I've tried everything. Opened every port ever mentioned for Xbox 360, turned on UPnP, put it in the DMZ. I've tried giving the Xbox a static IP and kept it on automatic... I have QoS set up but I am not sure if I have done so correctly. Any information on that part would be greatly appreciated.
I am running on cable, but do not have access to a modem because I live in an apartment complex where the cable comes straight from the wall, so I just treat it like I would a stand alone modem when it comes to routing. I hardwire from the wall to the router and to Xbox. Occasionally I am able to get Open NAT after messing with those settings long enough, but the next day it reverts back for some reason. Any ideas? I am currently just using port forwarding for TCP 80,3074,53 and UDP 88,3074,53 with a static IP for the Xbox.