For correct work of application:
1) In “Settings ⟶ Battery” turn OFF “Power saving” mode.
2) In “Settings ⟶ Battery ⟶ Launch” turn OFF option “Manage automatically” and turn ON options “Autostart”, “Work in background” and “Indirect start” for Prof Reminder.
3) In “Settings ⟶ Apps” turn ON option “Ignore battery optimization” for Prof Reminder.