Xbox 360 wireless adapter please Help!?

Everyday when I get home and turn on my xbox 360 it doesn't connect to xbox live and then when I test the connection is said I dont have enough MTU so I have to restore the settings on the router back to default then turn my wireless back on then it works.It never did this when I had a ethernet cable and it is getting really anoying.I have a Consolidated Communications router but I dont how to change the MTU.Can someone please help me so I dont have to restore my router everytime I get on.Also it wont disconnect me after I fix it is only the next day.