This is more often than not, a firewall issue.
Try forwarding port 80 of the NAS to your router, and then try accessing this externally (use your IP address to do this, found at ipchicken.com) : if it works, then it means that the FTP server isn't running on the NAS, and so the connection is being refused. So in this case, check your FTP config and ensure its activated.
If it doesn't work, then this is a firewall issue with your router, and you will have to consult google/the manufacturers instructions for more help with that as it is not strictly a DNS320 fault.