How to stop isochronic tone app to stop running 15 minutes after the screen goes ones dark

I have a Samsung galaxy tab s2 8 inch tablet. I have an app from Sine that has isochronic tone brain entrainmentsurgery in it. Many of them are supposed to run from 15 minutes to up to 8 hours. The isochronic tone stop 15 minutes after the screen goes dark. This makes the app virtually useless. I looked at app settings and found an option the says to stop tablet from going to sleep while the app is running.

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