I am having the same issue on my Galaxy S7 Android 7.0. Over the last week my whatsspp and Gmail apps have stopped working at some point everyday until i turn my phone off and on again and they work again for a few hours.
Not had any issues over the last year, always received notifications on emails and whatsapp messages. Even on muted conversations the number would appear next to whatsapp Icon to indicate an unread message.
I have read this thread and tried all suggestions but it still keeps failing at some point everyday, ill click on my whatsapp icon that says no new messages or my email icon and it will load messages that are a couple of hours old.
Sure enough, I turn phone off and on and then next few hours notifications work like normal. I have never needed to change all these settings on my phone to get notifications so dont know why i would suddenly need to now.
Any other suggestions anyone?