Please could you follow the instructions on this page and let us know if it fixes it:
https://www.androidheadlines.com/2016/01/how-to-fix-missing-notifications-on-huawei-phones.html
Then, please test notifications on this page again:
https://www.beelivery.com/Driver/Diagnostic.aspx
If it works, try rebooting your phone and testing again just to make sure it actually works even when the app isn't running.