WL-300g gets no DHCP response from cable modem

I'm trying to set up my WL-300g in Home Gateway mode behind my Comcast cable modem. I am able to connect directly to the internet through the modem with my laptop.

I've installed the latest (1.9.2.7-9) custom firmware for this access point, so I can telnet into it and run tcpdump to watch the network traffic. When I do this, after resetting the cable modem, I can see the DHCP requests going out from the modem, but no response comes back from the cable modem.

Does anyone have any experience with setting up the WL-300g as a Home Gateway on a Comcat cable modem? Or any idea why it wouldn't be getting a DHCP response?

Thanks!