Xbox won't connect to internet Via 360 test

Hello, I've recently purchased a new modem and router made by netgear I've been able to connect to the internet via laptop, cell phone, etc. but my real problem is that my xbox will not connect through the net work test when I physically do it myself but if my xbox is off and I reset the modem /router and then turn on the xbox it connects me but doesn't allow me to play any games or talk to friends, so back to my first problem I test the internet using the xbox, it connects to the network but not to internet and trows an error reset modem/router 30sec. ICMP error or MTU error I've browsed online for solutions but none are successful I've changed router settings bought an Ethernet cord
bought a second more powerful router from netgear nothing is working, any ideas?