Remonte IAX and Asterisk behind a Dlink doesn't work

Hi guys,
I have my Asterisk behind a Dlink DI-624 router and wanted to set up a IAX softphone on a remote location. I forwarded port 4569 on my Dlink to the Asterisk server, but my remote IAX just would not register. It registered fine on the local network but not on the remote network. I tried MozPhone, Kiax and Idefisk. All of them just timed out trying to register. Then I found some posts in Google about remote IAX and Dlink problems.
I found a Linksys router and configured it the same as my Dlink. The remote IAX registered immediatly.
I would like to keep my Dlink. Does anyone know what I can do to fix my Dlink to work with a remote IAX?

Have a great day,