Why can't I get Callback from Xero API Sample website

Question

I'm attempting to deploy the Xero Net Standard Auth2.0 sample app. But I'm deploying it not on my workstation, but on an Azure webserver.

The only thing I've changed is that (a) I've put the ClientId and ClientSecret into the appsettings.json file, and (b) for the CallbackURI in that file and Redirect in the config on Xero's site, I'm using the base URL plus /Home. Because just using the URL gives me an error when I click "Connect to Xero".

When I click "Connect to Xero", it takes me there, but when I click there to go back to the app, it just takes me back to the page with the Connect to Xero button.

What is supposed to happen is something like what appears in this gif:

https://user-images.githubusercontent.com/41350731/76293982-27636380-6306-11ea-8b83-e4dda46b7365.gif

Here's a snippet of code from the Home/Index.cshtml file:

<div>
    @if (Model == false) {
        <p>
            test
            <button onclick="location.href='@Url.Action("Index", "Authorization")'">Connect to Xero</button>
        </p>
    } else {
        <h5>API Sets</h5>
        <p style="color: grey"> You are connected to your Xero organisation. Try the following operations:</p>
        
        <h6> Accounting APIs: </h6>

        <section style="margin-left: 10px">
            <p><button onclick="location.href='@Url.Action("Index", "OrganisationInfo")'">Get Organisation Information</button></p>

Model continues to be false.

Any help would be wonderful, and gratefully appreciated.