Many routers provide an ALG on port 21 to listen on FTP commands to sniff out the port that needs to be open for active mode to work. However, that only works on port 21 in most causes.... so it doesn't work on non-standard
ftp ports.
Your can setup your FTP server for passive mode FTP (you'll need to forward an additional range of ports to the computer running the FTP server, specify within IIS to use those forwarded ports, and tell IIS to use
your WAN ip as the passive mode ip).