Hi,
Since you are visiting your VPN server from the Internet, you should configure the port forwarding firstly to map your VPN server to the public IP address, then the traffic from the Internet could be able to forward to the VPN server on the CPE router.
Please set the port forwarding under the navigation Advanced > Security > Virtual Server.
The LAN protocol should be the VPN protocol, and the WAN port is the destination port that you are going to visit on the Internet.