hi,
Consider 
http://forums.dlink.com/index.php?topic=10467.0 topic has not been posted in for at least 120 days, so i gonna post a new one here. 
i have the exactly same prob too. i already do the port forward by following this post 
http://forum.portforward.com/YaBB.cgi?num=1232511344 and a weird thing when i add saying "Since port 21 is used, the DSL router
FTP server port will be moved to 2121." But the E/I start/end port show is 21?
i also did setup a dynamic DNS server on dlinkddns. I can now access via ftp on LAN but not on Internet. 
My ftp server setting for DNS 323:
Max. User: 10  
Idle Time:  2 
Port: 
21Passive Mode: Use the default port range (55536-55663)
Client Language:  Western European  
Flow Control:  Unlimited  
SSL/TLS: Not Check
Any idea?
Thanks.