Bismillahir Rahmanir Rahim.
Assalamu 'alaikum wa rahmatullahi wa barakatuh.
To keep the screen awake, follow these step by step guide below.
Tap anywhere on screen to show the bottom menu and tap the Setting icon in the menu.
Tap the Screen Awake option as indicated by arrow so that it has the tick sign.
Jazak Allah Khair