Assuming everything is correctly configured, network, ip addresses, etc. you might only need to configure port forwarding on your router.
You need to forward all connections made to you public IP address (router outside interface) and port (RDP uses 3389) to the internal IP address of your server and port.
This way you’ll tel your router that every connection made to its internet interface, should be forwarded to your server. If this isn’t configured, the router can’t guess where to deliver that connection…
Check you router configuration about port forwarding, and you might get it done. If you don’t you can try to tell us the model, so we can help a little more.