Undead World: Hero Survival is able to automatically notify you when certain events occur in the game, such as your Idle Rewards being ready for collection. You can individually control specific types of messages to suit your preferences.
Access your Profile menu by tapping the profile icon on the top left of the home screen, then tap the Settings tab at the bottom of the profile screen.
On the Settings screen, scroll down to find Mobile Notifications.
You can toggle ON and OFF each type of notification.