Thank you for your asking. Please try the steps below to lock your screen in an app, aka "Screen Pinning":
>To turn "ON" Screen pinning
Settings>Security & location>Screen pinning>turn "ON">recent apps button (Square one of the 3 main buttons in the bottom)>choose an app you want to keep staying, press the top icon>Pin
>To turn "OFF" Screen pinning
Due to the system difference, ZenFone Max Pro doesn’t have the function so call "Child Lock" or "Guest Mode". However, you can try to enter "Safe Mode" to exclude the disturbance from the installed 3rd party apps.
Long press 2-3 times "back button & recent apps button" at the same time