SAMSUNG (Starting with Android 9):

1) Battery -> Optimization -> 'show ALL apps' (at the top) -> set THIS app to NOT OPTIMIZED

2) Settings -> Device Maintenance -> Battery -> Menu -> 'App Sleep' and 'App Standby' -> SWITCH OFF

* If not used for 3 days, Samsung will put the alarm app to these lists and then the app cannot wake you up anymore!! *

3) see: https://dontkillmyapp.com/samsung