Just follow the four steps and you'll soon be getting notifications as soon as your messages arrive.

Step 1:

Go to settings --> advanced settings --> battery manager --> protected apps, then find the PetBacker app, and "protect" it. This is essentially "whitelisting" the app, aka telling Oppo, "hey, this app is useful, please don't kill it."

Step 2:

Go to settings --> apps --> advanced --> ignore battery optimizations, then find the PetBacker app and ignore it. Don't be tricked by the misleading wording, "ignoring" the app actually means allowing it to run, because you're telling the battery optimization function to "ignore" that app.

Step 3:

Go to settings --> notification panel & status bar --> notification center, then find the PetBacker app, then activate "allow notifications" and also "priority display". You have to activate the priority part too to make sure you get notifications.

Step 4:

Open the app from which you want to receive notifications, hit the square (aka 'app overview') button on the navigation bar, then pull down on that app to lock it into place. From here on out, even if you're "clearing all apps," the ones you've locked will remain.

You have to complete all four steps to ensure you get full notifications from the PetBacker app.