Implementing Error 404 handling

I have a wisej.net SPA website. Built with wisej.net v 3.5.23. target .NET Framework v4.8. C# v7.3
Need to implement error404 when address is manually changed.
Error404 handling works now when address manual change includes a #, i.e. http://localhost:53672/#home
is changed to http://localhost:53672/#banana (thanks to the deeplinking info from this forum).
This correctly displays my custom 404 error page.
But when someone enters http://localhost:53672/banana it is not triggering the custom error page.
Microsoft CoPilot AND ChatGPT BOTH tried and failed to solve this:
Structure of website is as follows:
The home page is a Wisej Page with an empty panel and menu bar.
Name is MainShell. ALL content pages are user controls and are swapped in/out
as need via either menu choices/linkedlabels/buttons.
There is a NavigationManager class that handles all choices.

The changes both AI’s suggested result in either 404 or 500 error for wisej.wx and/or Default.aspx
Surely someone else has had this same issue and can tell me what I need to look at and change in web.config
or other code to get this to work. I can post code when I know what you want to look at.