Step01. click on "Settings" icon.
Step02. click on "General".
Step03. click on "Network".
Step04. click on "VPN".
Step05. click on "Add VPN Configuration...".
Step06. On the tab "L2TP" fill in Description, server, Account, Password, Secret, and turn ON " Send All Traffic". Click on " Save" button.
Step07. Turn ON VPN.
Please note that iPhone will disconnect VPN when it is not in use (When you are not surfing the Internet). Also, whenever you change cellular network, iPhone will drop VPN connection and you will need to connect manually again. Please complain to Apple to fix these bugs.