Motorola SB6121 and Netgear DGND3700 v2

Hello everyone! I just had my cable internet hooked up yesterday along with my Motorola SB6121 cable modem I got in the mail from an online order. When I hook up the cable modem to my computer directly, it works perfectly fine. However, when I attempted to put my router, the aforementioned DGND3700, into the fold, the connection went kaput. I have tried unplugging both the modem and router, plugging the modem back in, waiting for the link, then plugging my router in, and that doesn't work. I also have tried pressing down on the reset button on my router for 30 seconds to see if it just needed to head back to default, and that doesn't work.

I know it's not the regular ethernet cable because not only did the cable work going from the modem to the computer, but it also worked going from the modem to my PS3. I also tried using two different ethernet cables going from my cable modem to my router's WAN port.

Do you guys have any ideas on how I can get this to work? It's so frustrating, and I think it goes without saying how indispensable WiFi is. Thank you very much for any help you can offer!

EDIT: Updated thread title and text to mention a different modem I am using. Gave the other one to my family for their service.