As you say, my NAS is in a home network, where each device has an IP similar to 192.168.1.xx.
My NAS is 192.168.1.99. This IP address is forwarded to port 2121 in the router but the router will read it as 21. This is set, because I'm having several servers on my homenetwork, therefore, when connecting to the IP adresses with port 2121, my router knows what server (my NAS) I wanna connect to, and "redirects" to port 21 for FTP usage.
So my IP when connecting from outside is 91.144.xxx.xx and I wanna call port 2121.
Now, in XBMC which I have installed on the Apple TV, I wanna connect through FTP (because I'm not at home but still wanna have access and stream my media). So I fill in my IP, port, username and password to a networkconnection with the FTP protocol, but when saved in XBMC and I try to connect, I get an error.