I had this problem with OSX (Cyberduck) not connecting to via Asus WL-520GU router, and it got fixed by setting up 'Virtual server' tab in router settings as Port Range:22, Local IP:[N900 local IP], Local Port:22 and turning power save of the WiFi Network on N900 itself off, otherwise N900 couldn't be pinged from outside (and SSH to the device worked only when pinging FROM N900). On my other configuration with another Asus router, WL-500GP, things always were good. But that configuration has got a fixed IP, whereas this one has not.