How can I download a report? Credential problem

I'm trying to download a report (SSRS). More specifically I want to save it in a folder to be able to print it.

It turns out that in development I do not have any problems downloading the report. But at the very least I have to specify that I want to use the credentials by default:

using (var client = new WebClient())
{
    client.UseDefaultCredentials = true;
}

However when I try to publish the application to the IIS server I have to specify the credentials (username, password), otherwise I get an Error 403 Forbidden or 401 Unauthorized :

client.Credentials = new NetworkCredential("username", "password");

In my perspective, the IIS Application Pool user should have sufficient permissions to download the report, since I have it too.

This is my current code, without the print part.

public string Print(string reportUri, string printerName)
{
    using (var client = new WebClient())
    {
        var path = Path.ChangeExtension(Path.GetTempFileName(), "pdf");

        client.UseDefaultCredentials = true;
        client.DownloadFile(reportUri, path);

        return path;
    }
}

What do I have to do to download the production report without specifying the credentials?