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.
System Settings -> Advanced -> Recent App Management -> Normal Clear should be set, not Deep Clear
Apps -> Settings -> Apps Auto Launch
--> Turn this on in the top right corner and turn it on for COL Reminder