Zyxel 650R-31 + IPCop + VNC +ssh

Hello everybody!
I have a dynamic ADSL 384 with zyxel 650r-31 as a modem.
The zyxel is connected with the RED interface of the IPCop.
And some pcs on the green interface. (and an ORANGE Interface unused for now)
I use dyndns say myhost.tatata.org and i am trying to connect to a pc on GREEN interface running VNC (port 5900).
On the zyxel i put NAT settings to forward 5900 port on 5900 port at RED interface. And then IPCop to forward 5900 from the Default IP (the RED interface) to the pc on GREEN Interface. but it's not working ?
Any ideas what could be wrong?

P.S. If i ever manage that then i want to try through ssh ...

ok. it was working but i had to "ip nat loopback on" to CLI mode of the @#$@# zyxel. took me two days...

P.S. If i ever manage that then i want to try through ssh ...

You may want to do that. Not a good idea to use "raw" VNC over the internet.