Ichat & Zyxel Router


Hi there,
I have the following problem:
I have a router ZYXEL 623R-T1 and I tried to get my iChat working again...but somehow it doesn't work.
I tried everything in the preferences of the zyxel incl.
opening all sorts of ports.
nothing worked.
iChat connects with the server and I can use the chat function ...but any audio/video connection wont work.

I checked any available source ...I couldn't get an answer..

maybe here someone can help me.

I would love to use I chat again.
the funny thing is...at the moment I use skype and it works.

whats wrong with IChat?