I've setuip literally hundreds of environments just like this and they all work so I'm not sure why you say it has a 1% chance of success. The OP seems to have it working too.
Were you using active or passive ftp? You didn't specify and you also did not instruct the OP to configure for passive ftp - which is why I said it had a 1% possibility of working.
Active ftp will fail 99% of the time when setup as you described - with the 1%, as I indicated, being when the client is not behind a NAT firewall router.
ftp is different from most protocols in that it uses two separate communication channels - a control channel established from client to server, for which the default is port 21, and a data channel - traditionally, the data channel would be established from server to client and this is where the problem occurs. When the client is behind a NAT firewall/router, if the control channel is established on port 21, the firewall router knows to forward the incoming request from the server to the requesting client, rather than to block it. When the control channel is established on a non standard port this "fixup" does not occur - and this was the reason that the OP did not get beyond the username & password request - he already had the non standard port forwarding setup.
Passive ftp was developed to work around this problem - and the difference between the two is that with passive ftp, the data channel is also established from the client to the server, based on a port number sent from the server via the control channel. For passive ftp to work, it requires forwarding additional ports to the ftp server - which is what was missing from your instructions, and why it was unlikely to work.
Incidentally - prior to the release of the 1.08 firmware, the first release of the firmware in which the passive ftp port can be defined, it was not possible to use the DNS-323 with passive ftp from the internet - I therefore have to assume that the literally hundreds of environments you have set up were not "just like this"- in that they did not use a DNS-323 as the ftp server.
If you have the time to spare, search these forums and see how many people have struggled to get ftp working on a non standard port.