We need more information.

Distribution?
Modem make?
Router make?
Relevant init scripts?
DHCP client server?
Subnet setup?
Packet capture (would make this a hell of a lot easier)?

Basically, unelss we have more information, we will be throwing wild guesses at you, which would waste everyone's time. Each distribution, for example, keeps configuration files in different places, so we have no idea where yours are. Some routers or modems may have specific functions which you need to enable/disable. Perhaps someone here has the same one and has encountered the same problem. What you are asking is akin to telling us your car will crank, but not start, and then expecting a complete diagnosis. Is it the battery? The spark plug wires? The fuel injectors? The air intake? (you get the idea...)

Anyway, good luck with your problem. Post some more detailed info; zip it up if there's too much of it.

I hope we can get you online.