I am having the same problem as of yesterday morning, direct connection to the modem works but through the router doesn't. The router appears to get an IP through ipconfig, but through the DD-WRT config it doesn't have a WAN IP. Here is my post on another forum.
said by FearSonic:My internet connection and computer are both fine, otherwise I would not be able to type this at the moment. I am connected directly to my modem. Last night, my internet and router were working fine together.
This morning, I wake up to find that my internet was not working. I reset the modem and router, connecting the router only when the modem is fully initialized but no success. I then found that if I connected directly to the modem, it would work. But others in my house need to use the computer as well, so I'd like to use the router again.
Linksys WRT54G
DD-WRT v23 SP1 firmware
Motorole SB5100 SURFboard Cable Modem
If any other information is required, I will try to answer them to the best of my ability.