Error Code: 800 The remote connection was not made because the attempted VPN tunnels failed. The VPN server might be unreachable. If this connection is attempting to use an L2TP/IPsec tunnel, the security parameters required for IPsec negotiation might not be configured properly
Error 800 is often caused by GRE not being forwarded or processed. GRE is protocol 47. GRE needs to be forwarded, or the firewall needs to be PPTP- or VPN-aware (often called “VPN passthru” or “PPTP passthru” in firewall settings).
Port 1723 is done to exchange some basic parameters for the connection (the “control channel”), but GRE is the protocol used to transfer the encrypted data. So if only PPTP (port 1723) is working, you get stuck in-midst of the “registering” or “login” phase. Make sure that port 1723 is properly forwarded and the firewall on the router is not blocking any connections to the VPN Server. You also need to ensure that your router allows GRE 47.
Some routers do not support the GRE protocol, if that is the case then you need to replace your router with the one that allows GRE 47.