Android 9 does not show notifications from WhatsApp and Viber

After updating my Nokia 5.1 Plus from Android 8.1 to Android 9, I noticed that after a while I am not receiving notifications from any messengers like WhatsApp or Viber. This should not happen!

Method №1 (Official)

To fix this you need to perform 4 steps:

1) Turn "Adaptive Battery" on.
Settings → Battery → Adaptive Battery → On

2) Specify applications that do not need to be optimized.
Open "Settings → App & notifications → Advanced → Special app access → Battery optimization → All apps" and then open needed apps (e.g. WhatsApp or Viber) and choose "Don't optimize".

3) Check app's notifictions settings.
Open "Settings → App & notifications → <Needed application> → Notifications" and make sure that all necessary options are turned on.

4) Reboot the phone.