How do I fix WhatsApp not working on my iPhone 11 Pro Max?

In addition to the other posts:

Do you have a 3rd party VPN installed, even if you are not using it? If so, delete the VPN profile in Settings/General/VPN & Device Management/VPN. Don't just turn it off; delete the profile. If that fixes it, you can try reinstalling VPN. 

Standard user troubleshooting start with power cycling the phone. Test. Didn't work, the next step is a Force Restart, Force restart iPhone - Apple Support Test. Didn't work, the next step is to restore from a backup. Restore your iPhone, iPad, or iPod touch from a backup - Apple Support Test. Didn't work, the next step is restore to factory, Restore your iPhone, iPad, or iPod to factory settings using a computer - Apple Support Test. Didn't work, since this is an app, contact the app developer for support. How to contact an app developer - Apple Support

For support with third-party apps, contact the developer. Visit WhatsApp.com