ERR 596 SERVICE_NOT_AVAILABLE at ETClient()

Question

I have a very basic Installed Package, with an API Integration component...

...and a very simple connection test...

    public void TestConnection()
    {
        ETClient client = new ETClient();
        Console.WriteLine(client.ToString());
        Console.ReadLine();
    }

...with the following configurations...

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <section name="fuelSDK" type="FuelSDK.FuelSDKConfigurationSection, FuelSDK" />
  </configSections>
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
    </startup>
<fuelSDK
    appSignature="none"
    clientId="************************"
    clientSecret="************************"
    authEndPoint="https://mc**************************.auth.marketingcloudapis.com/"
    soapEndPoint="https://mc**************************.soap.marketingcloudapis.com/"
    restEndPoint="https://mc**************************.rest.marketingcloudapis.com/"/>
</configuration>

I have tried using both the config file, as well as adding the parameters to a NameValueCollection, and passing said collection to the ETClient method. I have also tried the proposed solution here -- without the addition to the authEndPoint, the exception is a 596 (service not available). When adding the additional, it's a 400 (bad request). I'm assuming this is something relatively minor, that I've missed, in the setup but I've yet to find it (or any documentation that goes into enough detail). Please let me know if you've been able to work through this issue in your experience(s).