Well then, there you go. You'll have to research turning off all the routing in the Comcast unit and just have it pass the connection to the Belkin router. Turn off the wifi on the Comcast so that it doesn't conflict with the Belkin.
Strangely, this looks like an older unit that doesn't have easily found documentation. I located a site that claims to have the pdf manual, but it wants to install an exe file on my computer to view the file - don't think that's going to happen on my watch.
In any event, if you plug a PC into the Ethernet port you should be able to open a command prompt and type ipconfig /all in Windows and find the base ip address of the router - probably 192.168.x.x usually 1 or zero for the x positions. Once you know the unit's address you should be able to just type the address into the attached PC's browser to get to the unit's admin pages. Once there, turn off wifi and try to figure out how to turn off DHCP and have the unit just act as a pass through to the Belkin unit where it can handle the network addressing and firewall chores (port forwarding). This should give you the control over your network that you want with more flexibility and connectivity using the Ethernet ports on the Belkin if needed. Once the Comcast unit is set up as a dummy pass-through then wire it to the WAN port on the Belkin unit and access the Belkin admin pages with a PC to administer your network and control ip addressing internally and port forwarding if needed to connect to the Subsonic host machine.
HTH - Let us know how you make out.
EDIT: Just follow these instructions!
http://forums.comcast.com/t5/Basic-Inte ... d-p/740333It doesn't look like this unit is anything more than an access point. I don't see any references to wireless service for the unit. Your should be able to plug it into the Belkin and hit the reset button on the RCA modem and have it connect.
http://www.dslreports.com/forum/r252997 ... s-WRT54GS2