Following are the specs for the XBOX 360. If you have one of the newer models with 5 USB, then yours does not support 5Ghz. You could get a 5Ghz bridge and connect to the wired ethernet port.
2.4 GHz wireless, 3 × USB 2.0, IR receiver, 100 Mbit Ethernet
Add-on: Wifi 802.11a/b/g, Wifi 802.11a/b/g/n
Revised "S" models
2.4 GHz wireless, 5 × USB 2.0, IR receiver, 100 Mbit Ethernet, Wifi 802.11b/g/n, AUX port
BTW, the DIR-825 has proved to be a pile of junk for me. I have a B1 version and upload speeds have been deplorable. I've tried every firmware imaginable including DDWRT. Dlink replaced and it and new one did not help.
Dumping the 825 saved me a few gray hairs (for now).