In the new Oreo OS, notifications and badges are interlocked. For that reason, if the notifications app is disabled, you will not receive any alerts for incoming messages.
If you want to be notified when you receive a message:
- Open settings and select “Apps”
- Select the application from which you want to receive “Notifications”
- Tap “Notifications”
- Tap “App Notifications” and slide the switch to activate it.