External FTP only working on port 21

[copycat]

I've created an FTP by hooking up a usb drive. When I enable ftp on port 21 I can acess it from WAN. However, when I change the port to something else (e.g. 21000) I can't connect from WAN, only LAN.
I've tried a virtual server setting to redirect the higher port to 192.168.1.1 but that seems to have no effect. Am I forgetting something?

[Antiloop]

tell us more information please

firmware version etc..

you should also look at this URL: http://grc.com/x/ne.dll?rh1dkyd2
fill in the port, and select 'User-Specified Custom Port Probe' to see if it's opened at all.