No fetching of instant messages and e-mail after update to Oreo 8.1
Based on this forum thread, this might be a known issue with your particular phone, and it looks like you've already troubleshot it pretty thoroughly.

Here are a few more things you could try if you haven't already:


Factory reset the phone
Follow all of WhatsApp's Notification problems troubleshooting steps. There are a few steps in there that you might not have tried yet.


Unfortunately, if all else fails, you might just have to go back to the previous version of Android that didn't have this problem.