I have try to setup like this but no luck.
I don't know why the IP Address for my VPN client is 192.168.31.0!
Have a look at the "Users" tab. Client IP addresses are defined there.
The other things is why the public IP XXX.XXX.228.123 to respond my VPN connection!
No idea to be honest.
Edit: I'm not sure how iOS determines which interface should be used for internet access.