A.    Disable Do not disturb and Power Saving on the phone
1. Open Quick settings panel.
Swipe down from the top of the screen to open Quick settings.
2. Turn off Power saving.
If it's illuminated, tap the Power mode icon to turn off Power saving mode. This could be preventing notifications from displaying.
3. Turn off Do not disturb.
If it's illuminated, tap the Do not disturb icon to disable this feature. This could also be blocking notifications.


B. Disable an app's power saving settings
If you have added any apps to sleeping apps, it means they have entered a powering save mode. If they are "sleeping," some of their functions may be unavailable when the app is closed. For example, they may not send notifications.
1. Navigate to Battery settings.
Navigate to and open Settings. Swipe to and tap Battery and device care, and then tap Battery.
2. Remove apps from Sleeping apps.
Tap Background usage limits, and then tap Sleeping apps. Touch and hold the app you don't want to sleep, and then tap remove.

C. Check if an app's notifications are turned on
1. Turn on notifications.
Navigate to Settings > Notifications > Included apps > Tap the switch next to the app.
Some apps allow you to choose what types of notifications you will receive. Tap the apps' name and make sure the notifications are enabled for your desired categories.
Many apps also allow more control of notifications when you tap on a category. You can make certain categories silent, choose what sound they make, or even allow them to still make a sound when Do not disturb is enabled.

D. More suggestions
https://www.makeuseof.com/tag/android-notification-fixes/