WhatsApp on the phone uses the network directly and ignores the proxy configured in the Wi-Fi. Putting the address in the settings does not change the IP the app presents.
Where it works
It works in WhatsApp Web opened in a browser that is going out through the address. There the browser is what the IP is seen on, and the configuration holds.
If what you need depends on the phone app, a proxy address is not the tool. Confirm that before subscribing to a package for this purpose.