Could you make sure UPnP (Universal Plug and Play) on the router is ticked "enable". If unticked, click the "check box" to enable UPnP and click Save/Apply and reboot the router.
Turn on your console and restore Xbox network settings to factory defaults. Navigate to Xbox network settings and select the Additional Settings tab. Select restore network settings to factory defaults. If wireless, re-configure Xbox wireless settings. If wired, press "B" on your controller and select test Xbox LIVE connection.
Post back and let us know how that goes.