How get notification when not login whatsapp?

Unfortunately it's impossible to receive Whatsapp notifications unless you are signed in. Notifications work by sending a text message directly to your device which triggers a notification; without login however, your device won't receive this text and therefore won't trigger one either.

Staying on top of messages may be challenging, but one effective solution is keeping Whatsapp open or checking it regularly. Or try switching over to another messaging service with notifications available even when not logged-in - some options such as Telegram and Facebook Messenger offer this capability so it may be worthwhile giving these a go instead.