Firstly, try these tips: If LW Will Not Connect (click on the blue link); (equivalent for FrostWire; if using the newest FW which I suspect you are, follow the LW 5 directions)

I'd like to see if the problem is the program or the router. I am aware many people do better port forwarding the SBG900, but let's see if it can connect properly otherwise. Also, are you running the SBG900 in wireless mode?

I'm not totally sure about the SBG900, but it does have a firewall section. Perhaps changing this to a lighter setting might help.