Access to ftp server from wan problem

I can’t access my ftp server from wan. Its working fine from lan. Anybody have an idea what I do wrong?

ZyWall (dns client to provider)
LAN 192.168.1.10 /WAN: something from provider.

ASUS WL-500 gd (dns client to ZyWall)
LAN: 192.168.1.44 / WAN: 192.168.1.45

There is no problem accessing servers ‘behind’ ASUS (if the correct routing is done on both Zywall and ASUS), but I can’t access the usb disk connected directly to the ASUS router (Webcam works fine from WAN) … and needless to say: I’m not an expert :^) Regards Nieman426

ASUS WL-500G Deluxe
Firmware: 1.9.2.7-6b