It is difficult to know what your problem is based on the information you've given.

The default port for FTP is 21. Your router will need to accept this port and pass it on to whichever computer you've installed the server software on (I don't and haven't used a router other than W2K so don't know how automagically yours does this).

I use Serv-U (Shareware - try for 30 days) which didn't seem too difficult to set up (W2K server (IIS) FTP server is another option but that /is/ a git to set up).

Mark