Follow along with the video below to see how to install our site as a web app on your home screen.
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perhaps there is an app that is closing the screen?
that could happen even though settings has it set longer.
Go to settings>security>cog, screen lock...that will bring up several times you can enter to screen from sleep. Set the time you want exit out of there, go back to display open sleep timeout and the list of Tim choices should include the time you entered in security settings. Set your new time. Worked for me... Good luck all