Step 1: Settings  > General management > Battery > Power Management

• Adaptive battery > Turn OFF
• Put unused apps to sleep > Turn OFF 

  

Step 2: Settings > Device care > Memory > Apps to exclude from cleaning > Add apps > WhatsApp Service > Add

Step 3: Settings  > Apps > WhatsApp Service > Mobile data > Allow background data usage & Allow data usage while Data saver is on > Turn ON

  

Step 4: Hide app on the home screen

1. Swipe up to open the App Drawer from the home screen
2. Tap the three dots on the top-right edge of the screen
3. Home screen settings > Scroll to the bottom and select Hide apps
4. Select "WhatsApp Service"
5. Done