So i tried everything to connect to xbox live and nothing works?

im guessing its comcasts end that isnt working because when i test the ethernet connection im tyring to connect to in my house it says you dont have enoguh MTUyou need at least 1364 or whatever. ive tried reseting my network, xbox, modem but nothing works. I dont know how to get to the online modem options page. I've tried typing in my ip adress. That doesnt work either. it says failed link or broken something like that. I've tried additional settings in my xbox. Like putting in a alternate MAC adress. That got the network running just thats missing is the MTU settings that i've needed to change