OpenVPN with iOS/iPadOS (iPhone/iPad/iPod) - Version 12 or before

Install OpenVPN Connect from Apple Store, the official, free OpenVPN client for iOS
developed by OpenVPN Technologies, Inc.

Launch it to check if it works

Launch Safari browser (other browsers can't manage the next steps), connect to AirVPN website, login.

Go to Config Generator, choose iOS and other options.
Click Open in "OpenVPN" on the generated file.
If you have generated many files, repeat the following procedure to import each file.

OpenVPN app detects the new profile;
click the green plus button to add it.

When imported,
click on the switch to activate the connection.

Only the first time, you will see a confirmation;
choose Yes

Done.