Are you trying to gain access to the NAS or web server from a remote location?
I know have use D-Link DNS NAS and to access FTP, i'm only using port 21.
Thats one test you could try, set up port 21 for FTP and see if remote FTP works, if it also fail, I presume there is a configuration issue somewhere or something in FW.
If this fails, I recommend that you phone contact your regional D-Link support office, ask for level 2 or higher support.
Please let us know what they say.