You've got it otherwise correctly, passive mode uses usually ports from 1024 for data connection and those should be forwarded to correct computer on LAN, but I guess the problem is, that your ftp server tells it's LAN ip address to client ftp. Therefore it only works in your own LAN. To be working, it should tell your router's public IP address instead.
The solutions which comes in my mind quickly... You must have ftpd which would have configurable IP-address setting for passive mode or it would dig out it automatically. Or just use active mode, but then client should have those data ports open. Or connect to internet directly from your server computer instead of using router (MiamiDX can do PPPOE).
The best solution is to have ftpd which digs out the IP-address, but I don't know if any Amiga/MOS server can do it out-of-the-box. BUT it's possible to add such feature to some clients... I'm using wu-ftpd on my Amigas and Pegasos and it has sources available on Aminet. I've added that feature to it myself, so it works behind the router from all computers to outside world. I've also compiled it separately for each computer so that dataports won't overlap and I know which ones to forward from router.
Edit: proftpd seems to have "MasqueradeAddress" setting, which should do the trick. There's a port in Aminet, I'd try if the setting works with it.
