Some things I would look into are:
I assume you've logged onto the router using an admin id & password and double checked the config - especially the wireless settings?
Have you checked the logs of the router to see what's happening?
Try re-connecting to the modem directly (wired) and go to
Canyouseeme and see what IP address is being assigned to the modem.
Have you setup an encrypted passphrase for the wireless connection and ensured it is the same on your Mac as your router?
When you connect your Mac to the modem directly is the assigned IP address the same as the one from CanYouSeeMe - it should be?
Finally can you get a command line on your Mac? If so, try the following command: IFCONFIG when you are in various states of connection.
I've made an assumption here, how many LAN ports do you have?
If it's 1 then use IFCONFIG eth0, else cycle the command through eth1, eth2 - increase the number for the number of LAN ports you have.
BTW: IFCONFIG is short for interface configuration and tells you a bunch of details about your network connections including the assigned IP address. If you're unsure of the command put it into Google first.
Hope some of this can help,
Steve.