This article explains How to Set up VPN on iPhone.
Find VPN setting
Select " Settings " > " General " > " VPN "
Add VPN Configuration
Type : L2TP
Write the description of this connection vpn, for example : My VPN.
Enter the ip address of vpn server , for example : 188.8.131.52
Enter your vpn's account and password.
Enter secret code.
Click " Done " to save your settings.
Before connecting, you must check your internet connection status, switch to connected mode.
From "connecting..." to "Connected", click "My VPN", you can see the VPN information after the connection is complete.