So you have two ways to receive your notifications : with or without Google services called Google Cloud Messaging (GCM).
Some apps need absolutely GCM to get notifications. In that case, using the GCM feature or MicroG is necessary. Here is the solution to make it work :
You will have to install WhatsApp again. If not, WhatsApp won’t be registered in MicroG.
Some apps have their own notification system when GCM isn’t detected (or if the app doesn’t want to use google services), like Signal and WhatsApp.
In that case, the app needs to work in background. So disable the battery optimisation of the app in Battery setting, and in the Trust section (Trust in Oreo, it’s called differently in Nougat), leave the “Start at boot” and “Keep alive” switch on for WhatsApp.