I need an android app to run in the background to utilize Simple Android Notifications

I'm making a mobile companion app to a web game and I need it to send a notification to the user when they are being attacked. I'm using this https://assetstore.unity.com/packages/tools/integration/simple-android-notifications-free-68626 to create the notifications, and I"m pulling my hair out trying to make sense of how to get this working in the background. I have a script that speaks with the server to get information on if the player is under attack or not. How can I make this all run in the background when the app is minimized? Or at least the parts I need?