Comcast is isp modem problems mtu settings and xbox live?

Hooked my xbox directly to my modem which is a surfboard SB5101U IT'S CABLE MODEM I'm pretty sure I should be able to sign right in to xbox live but it keeps failing saying something about the mtu settings I've never had this problem before and I've played on Comcast for months before I just got it again recently and wanted to play xbox live wondering how I can fix this. Oh and I don't have a computer at the moment so I can't do anything that way for a while so pleas help me thank you