problems reaching a natted ftpd

Hi
I'm running a pureftpd in my lan on linux 2.6 on port 21. So I want this
server to be reached from the Internet through my Internet connection.
Trivial you may think, but i got problems:

The ftpd is running on port 21 but my NAT-Firewall (IPtables/shorewall) is
redirecting from 9921 to 21 because my ISP blocks incoming on 21 if not a
registered host.
From here on it is possible to access my ftpd from Internet-side, but only
if the client-computer has an own public IP-Adress. If the client itself is
behind a NAT-Router it is only possible to login, but actions like LIST or
something are not possible.
The Error Message at the client-host then is something like "no route to
host".
Does someone got any Ideas...? Thanks in Advance!

Jochen