I received my unit a few days ago and have the exact same problem - my G300NH, regardless of firmware (and I can't flash to DD-WRT because I have the AO A3 version), will not reliably get an address from Comcast. Moreover, as others have experienced, I can after numerous power cycles get a DHCP address, but it's lost at the next DHCP lease renewal. I have also copied the gateway and DNS addresses and set the button to "router". I have a high level of technical expertise.
From what I read on other forums, the method used by the G300NH to get it's address is incompatible with the method used by Comcast.
I have repeatedly verified, using other wireless routers (including a Buffalo g router), that the problem is the G300NH.
Further, as shown on the small net builder web site, the G300NH has an unstable signal even on a clear channel - it will drop all the way down to 1Mbs, then back up to 54Mbs, for example. The signal cut outs have not been fixed by the 1.72 firmware.
This product is defective and not suitable for sale. I'm going to RMA the unit in exchange for a different brand router.