wireless

I will try to give as much info as possible. For starters Ihad to reset my desktop to factory settings. All was fine til I had to do this. I reinstalled my DSL, but had to enter the ISP address to gain access to the internet.
I then began to install my Linksys router, it went through all the set up until it was checking for internet connection. Said could find no internet connection. All lights were showing as should be on each device, DSL modem and router. If I disconnect router and just reconnect DSL modem back to computer I have internet.
I have reset the router to factory settings and tried but same thing. It detects the router but still no connection to the internet. What could be the problem????

Thanks