I upgraded my Microsoft Lumia 540 to Windows 10 Mobile. Now, my phone is not getting WhatsApp notifications, and I am not getting messages. It shows reboot your device. I tried to reboot my device more than 20 times. There is no change
Check in Background App settings if it is prevented to run in Background. Allow it to run in Background. Also check the software version & firmware of your phone. Update them both to latest versions. Also the whatsapp app should be updated to latest version. The problem may go away.
Are you running your phone on 'Battery Save' Mode? If yes, like the answer above me. Check in Background App settings. But, if your problem are unsolved until now, check the Store, is it an updates for Whatsapp? If yes, update it. If no, delete it. And trying to install it again, good luck :D