What exactly are you trying to setup? FTP or Web Access? Given that it works from your LAN does indicate that you have to do some port forwarding. Go into your router config and forward port 20 and 21 for bith UDP and TCP if you want to use FTP, regarding web access, go into the web access setup page on the linkstation -> disable UPnP and select a port number (10000) for example. Once done forward this port in your router and that should be it.