[moaningmags]

For some weird reason, some instructions say to power the router on first then the modem.

But turn it all off, turn on the modem, I'll assume it's an Ambit, wait for SYNC and RDY to go solid.
Turn on the router, wait a few mins then turn on computers.
9 times out of 10 this fixes the issue.
What IP address does your computer have?
Can you access the configuration page of the router. The address for it and username and password should be with the instructions.
There should be a page in there that will show you the IP address the router is getting from the modem.

HTH