I setup xboxs as I stated with a static IP and can set the MTU in your router settings... the ip address for the router should be on the bottom of it... you can bring up the configuration by typing this in a web browser ( example http://192.168.0.1 )...the username and password should be on the bottom of the router also if it has not been changed
Xbox Live requires a minimum MTU setting of 1364. If your console does not pass this test, you cannot sign in to Xbox Live.
If you have multiple consoles that are connected by using the same connection, you may cause a failed MTU tests. A failed MTU test may also be caused by sharing the connection with a computer.