How to disable the auto-lock option
To disable the auto-lock option, depending on the iOS version, follow the steps below:
iOS 10 and higher
Go to “Settings”
Choose “Display & Brightness”
Click on “Auto-Lock” and select "Never"
iOS 9.x and earlier
Go to “Settings” on your phone
Choose "General"
Scroll down to find the "Auto-Lock" option
Click on it and select "Never"
When iPhone is in “Low power mode”, it is not possible to set “Auto-Lock” to “Never”. You need first turn off “Low power mode”.