A reverse proxy uses one of request forwarding. The downstream router in this case did not initiate the request and so does not know to which internal IP to pass the request to unless you set up port forwarding. Tunnel services such as this solve this issue by setting up a persistent connection between the client and server and relying on the NAT features of your router. Because the client initiates the socket connection your router knows which private IP to forward packets to when the upstream service (e.g. Tunnelmole) sends data. NAT traversal means you don't need port mapping.
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A reverse proxy uses one of request forwarding. The downstream router in this case did not initiate the request and so does not know to which internal IP to pass the request to unless you set up port forwarding. Tunnel services such as this solve this issue by setting up a persistent connection between the client and server and relying on the NAT features of your router. Because the client initiates the socket connection your router knows which private IP to forward packets to when the upstream service (e.g. Tunnelmole) sends data. NAT traversal means you don't need port mapping.