I have that exact same router at the house.. the best thing I have found to d is not fight the router and let the computer with the FTP server be the DMZ host so basically and requests to port 20 or 21 will be forwarded straight to the computer put in the DMZ host. The drawback is that you will no longer be protected by the router's built in firewall so you might want to setup a program like Zone Alarm on the computer set to the DMZ Host..
I have a webserver running like this with no probs and also no probs using FTP or letting friends and family hook up to the file server and transfer files.. any other way except this I could not get to work plus these Belkin routers are bad to freeze up every once in a while atleast once or twice a day.
Check it out...
http://204.116.113.193/Website/matt.html
Now if it don't work then blame the router... already had to set it once today.
Ok now make that twice... Only buy Belkin Cables not routers... screww this I'm getting a Linksys.