Hi, i need yuor help

I have a V2800 and i'm not able to connect to my Lan HDD from external internet connection.
I did that:
open a port (for es. 21007) on "open ports" page (UDP and TCP) to local computer 192.168.0.3 (HDD IP address)
redirected port on "Port redirection" page (UDP and TCP) to 192.168.0.3 (public and private port 21007)

I try to connect on "xxx.xxx.xxx.xxx:21007" but it doesent work ... why?
If I set DMZ on 192.168.0.3 it works ...

pleeeeese, halp me !!!

ps = sorry for my bad english lang