I wanted to provide more context on OriginalGriff's message. I was working on an Android application via Xamarin a while back and was hoping to add notifications and was having a lot of issues, and they stemmed from the version of Android I was running.
Newer versions of Android (Oreo+) have stricter restrictions on background services, now shutting them down in order to save on resources and battery. This has caused headaches among Android developers, and the recommended practise for the newer Android versions is to in-fact create a "foreground" service which can produce a notification which sits in the notification drawer effectively permanently, allowing for background processing.
This might mean the phone will display a permanent notification (I've seen some Android applications do this, and I manually hide them to keep things tidy) but it will assert that your application will receive notifications. If you've noticed some applications don't create this notification, it's because some providers (ie. Samsung) white-list certain applications to ensure that the background services don't get shutdown.
Here's a link to a StackOverflow answer
] which gives a good explanation of what is going to be needed. It does target Java Android but you can use the basic principle to work out how to implement it in Xamarin.