OnePlus - Oxygen OS - No Notifications - App Hibernation

Please follow the steps from this blog post too:
https://www.forbes.com/sites/bensin/2017/07/28/how-to-fix-push-notifications-on-oppo-phones/#601399891735

And check this settings as described here:

1.)

System Settings -> Battery -> Menu -> Aggressive Doze & App Hibernation

--> Turn these off completely or at least select "COL Reminder"

2.) System Settings -> Battery -> Menu ->  High Power Usage apps

If COL Reminder is shown here, disabled them.

3.)

System Settings -> Advanced -> Recent App Management -> Normal Clear should be set, not Deep Clear

 

4.)

Apps -> Settings -> Apps Auto Launch

--> Turn this on in the top right corner and turn it on for COL Reminder