Do you run the Test Xbox Live test and if so is it failing in the IP Address with an address of 169.*.*.*?
If so do you have other wireless devices that can get an address from your Router?
First things to check would be if encryption is turned on and the Key for the encryption and Type WEP/WPA/WPA2 etc.
Next I would suggest assigning the Xbox a Static IP address (Manual) on the Xbox's Network Screen--Be sure to pick an address that is not being used on your network and is outside your routers DHCP range.
Make sure you have not set up Mac filters and if so be sure that the Xbox's Mac Address is allowed onto your network
If you set the Xbox up with a Static address be sure to turn the xbox off and then turn the router off for a few mins. Power the router on first make sure other wireless devices are working and then power the Xbox on.
Please click on the tab "Application and gaming" and click on sub tab "Port range forwarding" 1) On the first line in Application box type in "Xbox", in start box type in 88, in end box type in 88, in protocol keep it both, in ip address type in 192.168.1.20 and give a check mark on enable box. 2) On the second line in Application box type in "Xbox", in start box type in 3074, in end box type in 3074, in protocol keep it both, in ip address type in 192.168.1.20 and give a check mark on enable box. 3) Now save the settings 4) Once you return to the setup page, click on the tab "Administration" and disable the UPNP and save the settings. 5) Now assign the given ip address on your xbox ip address :- 192.168.1.20 subnet mask :- 255.255.255.0 default gateway :- 192.168.1.1 Please assign the dns address on the xbox primary dns :- 188.8.131.52 secondary dns :- 192.168.1.1 6) Please try to test your xbox live.