Open your phone’s Settings
Go to Apps or App Management
Select WhatsApp
Tap Mobile Data or Data Usage
Turn off Background Data
Turn off Mobile Data access for WhatsApp if available
Keep Wi-Fi on if you want WhatsApp to work only on Wi-Fi
On Android, you can also go to Data Saver or Network Access and block mobile data for WhatsApp
On iPhone, go to Settings
Tap Cellular or Mobile Data
Find WhatsApp
Turn off the switch for WhatsApp
Use WhatsApp only when connected to Wi-Fi