Xbox 360 mtu over 1364?

so im out of town and im staying at my cousins house and i brought my xbox. For the first couple of days xbox live was working perfect, but then for some reason it quit working. I did test connection and it said my mtu was not sufficient. So i do what it says and i go to my router settings and it was 1492 mtu. So i tried every thing... i tried 1400 1364 1480 and alot of other ones. The router is right next to the xbox and i even tried connecting straight to the modem and it still didn't work. idk if i connected it wrong but it would still say mtu not sufficient. All the computers in my cousins house connect and work fine... but my xbox isn't. its not the xbox either because i tried it on my other cousins internet and it connected. I searched google and i even tried connecting it to my laptop and getting internet. Anyone have the same problem? Any ideas or something to help me please!? Person who gets it fixed gets full points