Step 1: Settings SettingsBTN.png > Device maintenance > Battery > Unmonitored apps > Add apps > WhatsApp Service > DONE

Device maintenancebaterryUnmonitored appsadd appswhatsapp service

Step 2: Settings SettingsBTN.png > Apps > WhatsApp Service > Mobile data > Allow background data usage & Allow app while Data saver > Turn ON

  appswhatsapp servicemobile dataallow settings on