Outlook desktop notifications not showing up

Please confirm whether the new incoming messages are in the Inbox folder of the mailbox because new mail desktop only works for the default inbox.

You can refer to this thread to check if all the settings are correct:[Troubleshooting] New mail desktop alert not working

If issue persists, we can repair outlook data file to test the result.

Besides, for windows 10, there is an option called Quiet Hours in side tab. It will block all active alerts. If it is turned on, please turn off this option.

Update: 6/25/2021
Turning on Cached Exchange Mode in the Outlook settings and also right-clicking the EXE and Troubleshooting Compatibility to an older version of Outlook fixed my issue.

Original Answer
The way I got it working was to set up the Windows Mail application with my Outlook account. It still doesn't work in Outlook, but at least I can see notifications.