How do I put this over with out offending - going solely from your responses to my questions - I'd say you're more than a little off base.
I asked what ftp client are you using - your response appears to be "the DNS-323 ftp" - sorry - that is the ftp server, the client is the piece of software you're using to connect to the ftp server. Try using Windows command line ftp client - go to a command prompt and type ftp - you will have to issue the required commands one by one, but you will be able to see exactly where the process fails.
Next - forwarding to the correct IP address - which is why it works just fine on site - forwarding is not used for local access, that is more likely to be why it works just fine.
It is possible that your problem is caused by a firewall issue with the router - however - if the provider has configured the router to forward the port, he/she should also have known that it would be necessary to configure the firewall to permit access - depending on the type of firewall/router in use, this may be a one step or a multi-step process. With some firewall/routers, once the port is forwarded you're done, with others you must forward the port (in the router) and then allow the traffic (in the firewall).
Other (greater) probabilities are ....
- the client you are using is configured for passive ftp and not active ftp (which is what your DNS-323 & router appear to be configured for presently) - please see note below.
- the client you are using is configured for active ftp - BUT - the firewall at the client side does not automatically "fixup" the ftp protocol.
If you are using an ftp client that returns the error messages (the windows command line client does), we can pinpoint the location of your problem and then look for ways to fix it.
Note - the ftp server on the DNS-323 does not fully support passive ftp, it does work on a local network but it does not allow you to configure a number of parameters required for it to allow remote access using passive ftp - this will supposedly be fixed in the 1.08 firmware.