How to receive WhatsApp notification when away from iphone ?

Well it would work, when these conditions are true:

To receive SMS, MMS, or push notifications from third-party apps on your cellular Apple Watch, your paired iPhone must be powered on and connected to Wi-Fi or cellular, but it doesn't need to be nearby. You also need to be signed in to iMessage on your iPhone.

Use your Apple Watch without your iPhone nearby - Apple Support

But that is the catch, since there is no WhatsApp app for Apple Watch. Hopefully they will release support for more devices at some point.

Yes like I said, I think we'll need a true watchOS version of WhatsApp before that will really work out.