Probably a case, if you are connecting through a router from your pc at home to another over the internet, via a router it is not set up right. the config of the internal routers firewall is set wrong or you have not set up what we call port forwarding in the settings of the router. Look up port forwarding for routers, and router hardware firewalls. Look up what port number by default windows remote desktop uses. IE: port 8080 make note of the protocol it uses it can be TCP or UDP. You will have to log into your router if you have one and put the settings in the port forward option of the router Ie: Name of program. Protocol to use Tcp/Upd and the port range Eg: 8080 to 8096 ect. save the settings exit the router.
If the ports selected do not match on the computer you are trying to connect to. It will result in a failed connection. so you need to know what port address the other computer is using at the other end, the port setting for remote access to allow you to connect to it also.