Self-assigned IP address and cannot connect to internet

Hi everyone. This problem has been driving me nuts.

First, I have RCN cable with the modem that they supplied me.

Second, I want to connect my router (Westell 6100) to the modem so that I can have wifi.

Here are the different scenarios that I have done with their respective results.

1. Using ethernet cable, connecting from my modem directly to my MBP, i have internet access. So no problems there

2. I connect the ethernet cable from the modem to the WAN port of the router. Wireless is active as is internet (both lights turn green). Using my iPhone, i connect to wifi, it asks for the password, I enter it in, I am connected. Using my MBP, it says I have a self-assigned IP address and I cannot connect to the internet.

3. I connect the ethernet cable from the modem to the WAN port of the router. Wireless is active as is internet (both lights turn green). If I connect another ethernet cable from one of the LAN ports on the router to my MBP, I have internet access

So I guess my question is, why isn't my MBP picking up the wifi like my iphone is? If anyone could help me, that would be great. I have searched around the internet, but nothing has helped.

Thanks,
QP