UWP Application setting default proxy with default windows authentication

I have a UWP app that signs in using Azure AD. I deployed it to a customer site and it stopped logging in. They have a network with a proxy and I don't have access to their network, so I can't debug it properly. All the information I was provided is:

There's a proxy, you might want to use the default proxy settings. The proxy requires default windows authentication.

So with that information, here's what I've tried:

1. Update my PublicApplicationBuilder to use a custom HttpClientFactory

this.publicClientApp = PublicClientApplicationBuilder.Create(ClientId)
     .WithAuthority(Authority)
     .WithHttpClientFactory(new MsalHttpClientFactory())
     .OtherNonRelevantMethods()
     .Build();

2. I created a custom implementation of IMsalHttpClientFactory which attempts to set the proxy. There's a function, GetHttpClient which should setup (as the name says) an HttpClient object with all the necesary configuration

private HttpClient GetHttpClient()
{
     var handler = new HttpClientHandler();
     handler.UseProxy = true;
     handler.UseDefaultCredentials = true;
     return new HttpClient(handler);
}

This doesn't seem to work. It was my first attempt at using both the default proxy and default windows credentials.

I have also tried to manually set the proxy credentials with the following code

private HttpClient GetHttpClient()
{
     var handler = new HttpClientHandler();
     handler.UseProxy = true;
     handler.DefaultProxyCredentials = CredentialCache.DefaultCredentials
     return new HttpClient(handler);
}

But it seems to do the same thing, and therefore doesn't work.

And finally I tried to manually retrieve the proxy from WebRequest and Credentials from CredentialCache but again... no luck.

private HttpClient GetHttpClient()
{
     var handler = new HttpClientHandler();
     handler.UseProxy = true;
     handler.Proxy = WebRequest.DefaultWebProxy;
     handler.Proxy.Credentials = CredentialCache.DefaultCredentials;
     return new HttpClient(handler);
}

I'm unfortunately running out of ideas, I have been able to make this work by running it through Telerik Fiddler, so obviously Fiddler can reach the proxy just fine and when Fiddler operates as a man in the middle, my application works fine. But I can't simply ask them to run fiddler every time haha

Do you have any ideas? Am I missing something super easy?

Thanks