I am no expert but I read my server instructions and this
works for me:
If your FTP server has a public IP it can use port mode.
For port mode you need to open port 20 on the WAN interface
that the server uses.
If your FTP server has a private IP you need to use PASV
mode. For PASV mode you need to open a range of ports above
1023 on the WAN interface that the server uses. Your FTP
server software should allow you specify a discrete range of
ports so all ports above 1023 do not need to be open.
I have FTP servers in both situations so I have tested both
these routing methods.