There are two possible causes for this:
On newer versions of Android, the user must give the app permission to show a notification. Android will not allow the app to play in the background without a notification. Please allow the notification.
Many Android manufacturers are being over zealous about battery management. For the more zealous manufacturers, they are suspending the app after a few minutes in the background with the default settings on the phone. To resolve the issue you must making changes to the phones settings.   This causing issues for many apps.  See the following for more background on the issue: https://dontkillmyapp.com/
To change the battery settings for the app, go the phone's settings:
Select Apps
Select Reader
Under Reader's App Info, select Battery.
Select Unrestricted