Did you do a full factory reset? If the problem is still there it's gone bad. I have seen that happen with routers and Airports, all looks good but you can't get anywhere.

Can you connect directly to the modem with a cable okay? Have you reset your modem? Make sure you reboot the modem after plugging the cable in so it associates with the computer. Some modems only hand out one IP per ethernet port so you have to reset them when connecting them to a different port.

The cable for the modem is on the right port of the base station?