Use a different port in Snapshot FTP destination

I was testing out Snapshot Pro to create snapshots then send to my FTP server. It all works when using FTP on port 21. As I would expect. But I'd like to change the port to something else (just for the slight increase in security it offers). But when I try (using Port Forwarding on my router) it gives me a connection error. If that is something not possible, then that port box should not be available to change. It's very confusing if a box is there and allows some kind of change which then fails.