|Since your NAS is behind a router, you'll have to create a port forward on the router to the NAS device which will allow external connections to remote in.|
Since you're using ftp, your forward would look as follows:
forward Port 21 to [LAN IP of NAS]
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.