Cant connect to xbox live, Mtu to low need help?

Ok, I use to connect my xbox through a ethernet cord that goes directly to modem. A couple of days ago, I went to my cousins house and they have wifi. I set it up there and played for a week. I move back home and try to connect but it says mtu is to low. What's happening here. I trouble shoot and it says to try connecting directly to modem. But that's how I been connecting to it. I have DSL service and a speed stream 5100 modem. I know its compatible cause I use to connect to live with it. What should i do. Also the default mtu for a modem from att is 1500. IM so confused and angry. To me xbox live is like crack to a crackhead. I need my live man.