Hi

You must check several things.
1) Go to Settings, Apps, WhatsApp und in there to Permissions (check all) and to Notifications (check all). At least go to Memory and delete only cache.
2) Open the App Phonemanager, right above is the settings symbol. You can make shortcuts of the different parts on the Homescreen. You must open Akkumanager - it has also right above the settings symbol. In there activate: WLan on in Sleeping Mode and important: mobile Data on in Sleeping Mode. And in the main menu of Akkumanager activate WhatsApp as protected App.
3) Open settings, more (after datamanager), Mobile Network - mobile data always on.
4) Open Developer Options Mobile data always on.
5) Restart phone.
If it doesn't work, I give further instructions