If you have a newer version of Android, then you'll have to turn off any Power Saving apps or features. You can typically do this from the Device Maintenance or Battery in the Settings menu. After you turn off the power saving features, go to the Settings menu again and locate the Notifications. If you don't know where it's located, use the built-in Search features as most of the phones have one nowadays. Type Notifications then configure your application to be allowed to send you notifications.