networking – My Android Client can’t connect to OpenBSD VPN Server

I am configuring a point-to-site VPN (IKEv2)by using an OpenBSD IKEv2 server and an Android client.
I have tried to follow the instructions given in the following link : http://www.openbsd.org/faq/faq17.html
I am configuring my Android client via StrongSwan application.
After following the instructions, my client can’t connect to my vpn server which I think is normal because I configure only the public IP address of my server to the client VPN Profile.

My question is: How can my client connect to the VPN server only by knowing the public IP address when in fact there are many devices that have the same public IP address under the same LAN? (The VPN Client is also under the same LAN and has the same public IP address)