First, you need to make sure the remote system you are trying to access is running VNC server. Without that, you are out of luck.
Second, the router that the remote system is sitting behind, is allowing requests coming to port 5900-5910 range and directing them straight to the system you are trying to reach.
Last but not the least, the network your current computer is in, should be allowing outgoing connections to port 5900-5910
I am under the impression that you are trying to reach your desktop at home from a public wireless access point (like Starbucks) or from your work place. If you satisfy the conditions above and you know the dynamic IP address of your remote system (if you wish, you can use a free service provided by
no-ip.com or alike to circumvent this concern), you should be able to access your remote desktop.
I am not very familiar with remmina or vino, but using the freely available VNC-Server and vncviewer applications, you can accomplisy what you are looking for.