It could be because of the settings that are done on your device. You can change the below settings of your device in order to receive notifications.
Please go the phone's settings, look for 'Battery', click on 'High Background Power Consumption' and then enable 'PetBacker' app to receive the notifications.
Enable "Auto-Start" for PetBacker
Go to i manager --> App Manager --> Autostart Manager --> Enable PetBacker App
Settings --> More Settings --> Applications --> Autostart --> Enable for "PetBacker"
Click on App Manager
Click on Auto Start Manager
After following the above steps come back to :-
App Manager --> Tap on Uninstallation / Installed App --> PetBacker --> Enable Show Notification
Settings --> More Settings --> Application --> PetBacker --> Enable Notification