[Hampshire] using wu-ftp

Hello Folks.

I have installed wu-ftp on a debian etch machine.

I can ftp into /home/ftp as anonymous user using a laptop on the same
network without probs using the internal ip address of the ftp server.

I can't ftp into the same server using its dynamic dns address.

My modem router is forwarding port 21 to the same address that I can
access via the internal route so to speak.

Any thoughts why access is denied in the second case and not the first
please?

TIA
Mike