Microsoft Advertising API Sandbox OAuth settings

I am attempting to test the OAuth flow for a web application using the Microsoft Ads Sandbox. We've had this working in our application for years, but awhile back (like a few years ago), this functionality stopped working for us in the sandbox, but since things worked fine with the production Microsoft Ads API, we didn't do anything about it. We are in the process of reworking this OAuth code and would like to test it against the sandbox, but we can't because I'm getting this error:

We're unable to complete your request

invalid_request: The provided value for the input parameter 'redirect_uri' is not valid. The expected value is a URI which matches a redirect URI registered for this client application.

The reason we're getting this error is because the OAuth redirect urls have, in fact. changed due to the recent changes we've made to our application. The OAuth redirect urls are configured for applications like that through some sort of a "development center" application that allows developers to enter the valid redirect urls for their application. We have this configured for our production application, but we can't find we're to configure this for the Microsoft Ads API sandbox. I know that several years ago, I configured these settings for the MIcrosoft Ads API Sandbox, but I don't know where to configure these settings these days. I found this post: https://social.msdn.microsoft.com/Forums/en-US/d1b68bc8-e5db-4fe4-be5b-0dd0261e8639/cant-authenticate-with-oauth-in-sandbox?forum=BingAds that says that the settings can be edited here: https://apps.dev.microsoft-int.com/#/appList. However, when I go there, I get this:

I don't want to add an app in the Azure portal because I already have an app. Would the Azure portal even let me add a app for the Microsoft Ads API sandbox anyway (or would it just be a production app)? When I click the link to add and app to the Azure portal, I am unable to login with my sandbox account, anyway, so I can't even try to add my sandbox app:

The Azure Portal claims it can't find dev@finch .com, but that is definitely my Microsoft Ads API sandbox user. In any case, I'm very confused about how I am supposed to change the OAuth settings for my Microsoft Ads API sandbox application right now. The settings UI just seems to have disappeared with no alternative to replace it? It used to work, but now it says I can't use the old UI any longer because it's a "converged application" (whatever that means), and I can't seem to use the Azure Portal to do it, either. Can you tell me how I can change the settings of my Microsoft Ads Sandbox application?

Thanks,

Jon