Router cant find internet

Hello,
my problem is our internet works fine, but when i hook up our new Linksys router model# WRT300N the router shows all the lights it should internnet and everything but at the end of the instillation cd when it checks for internet it cant find internet even though the router shows internet. i tryed reseting modem and router but nothing happens I tryed turning fire wall off to see if it was the problem and still nothing. If you have a answer how to make it work just write to my email address please. ty