Not sure if this still a problem for you..
What you need to do, is edit the proftpd config. Normally ( /etc/proftpd/proftpd.conf )
Adjust to passive ports to whatever you want them to be.
Then you need to forward those ports to your FTP server
iptables -t nat -A PREROUTING -p tcp --dport 65000:65100 -i eth0 -j DNAT --to IPOFFTPSERVER
The eth0 part will depend on your setup. might be different
This worked for me, I was able to establish a connection with server and do a list.. I was also able to download files.. although I am still getting the Server sent passive reply with unroutable address. Using server address instead. error but it all seems to work? If anyone has any more information on this, please share!