1. Turn off the device
2. Now switch your phone back on. Immediately you see the Samsung logo appear on your screen, press and hold the Volume Down key until the phone boots completely to the home screen.
3. You should now see a watermark caption "Safe Mode" at the bottom left corner of your phone screen. This will now disable all 3rd party apps.
I have a different device (A Galaxy J7 Sky Pro S727VL) which has recently developed this identical problem of no longer getting Notifications for arriving Text Messages. A few days ago it was giving them out fine.
I have looked through all the Notification settings; have tried Safe Mode (problem still continues in Safe Mode), have looked at Power Settings, etc.
Nothing seems to solve this issue which only recently started a few days ago.
I have not recently installed any new APPS which would have changed things; although some Updates were recently applied to existing APPS within the Play Store.