MTU Setting Keeps Failing on XBOX 360!?

So I tested my connection and the MTU failed. I have a DL524 router by dlink and the motorolla surfboard modem. i havent registered the modem because i just got it idk if thats the problem. and i checked the MTU setting and its 1500 and the minimum is 1346 or something like that so idk i need help and i have verizon fios internet and phone but optimum tv idk if that helps but the tech support line was no help