There are two parts to the answer: 1) first, to do what you're asking, rather than try to reset the app specific battery setting, go first to the main settings then battery, and be sure that toggle ON Background app manager; then go back to the app specific setting and its battery setting for background app management will no longer be grayed out, so now you can turn it off. However, 2) this won't really solve your problem because an Asus built-in app called Mobile Manager in its default mode will over optimize and keep killing the app you want to keep running in the background by toggling off the Allow Background Usage switch for that app, exactly because it's running in the background. The Mobile Manager tries to help you conserve battery by disabling apps that run in the background. Well intended but not helpful when you want the app to run! So, the solution is to go to settings/apps & notifications and find the Mobile Manager app, and then go in and disable its allowance to run in the background.
