Configuring Sitefinity 10.1 to use an external STS

We need to use our version of IdentityServer4 as an STS for Sitefinity.  I have followed the steps as laid out in the document "Implement custom external identity providers." (docs.sitefinity.com/for-developers-implement-custom-external-identity-providers) I can trace the code in global.asax.cs and see that my custom provider is registered. However, when I click on the button on the login page for my external STS it is still using Sitefinity's internal IdentityServer. I know this because I see the client_id that is passed in the URL. I can go to Advanced Settings -> Authentication -> SecurityTokenService -> IdentityServer -> Clients and find the IdentityServer Sitefinity is using and disable it (uncheck Enabled). Sitefinity is still trying to use that IdentityServer to authenticate (and generating an error because it's disabled) rather than use the custom AuthenticationProvider that I have created and enabled. How do I get Sitefinity to use my custom AuthenticationProvider and ignore this other  IdentityServer?