Welcome to the Auth0 Community!
Thank you for being so patient, and sorry for the late response.
I’m not an .net expert so I’ve been searching for potential solutions for your question.
I would highly encourage you to check out our tutorial ASP.NET applications, Auth0 ASP.NET Core MVC SDK Quickstarts: Add Login to your ASP.NET MVC application
There’s also a thread by one of the community members about a problem with /callback. MVC Core 6.0 Trying to get the example work
Let me know if this helped you
Thanks and have a great day
Dawid