Sending Push Notifications to api.sandbox.push.apple.com fails

I am developing a new App that uses Push Notifications. I completed the development of the App itself and I can send Push Notifications from the CloudKit and receive it on my phone.

My problem is sending the Push Notifications from our service API to Apple. If I use the production address gateway.push.apple.com, I can send messages, but they don't work for my new App because it is in development and not yet released. If I use the sandbox address api.sandbox.push.apple.com, I get the following exception:

System.Net.Sockets.SocketException (10061): No connection could be made because the target machine actively refused it. [::ffff:17.188.143.98]:2195
   at System.Net.Sockets.Socket.DoConnect(EndPoint endPointSnapshot, SocketAddress socketAddress)
   at System.Net.Sockets.Socket.Connect(EndPoint remoteEP)
   at System.Net.Sockets.Socket.Connect(IPAddress[] addresses, Int32 port)
--- End of stack trace from previous location ---
   at System.Net.Sockets.Socket.Connect(IPAddress[] addresses, Int32 port)
   at System.Net.Sockets.Socket.Connect(String host, Int32 port)
   at System.Net.Sockets.TcpClient.Connect(String hostname, Int32 port)
   at System.Net.Sockets.TcpClient..ctor(String hostname, Int32 port)

The service API for sending Push Notifications was developed in C# .NET 8.0.

Constants:

    private readonly string _hostname = "api.sandbox.push.apple.com";
    private readonly int _port = 2195;

The code for sending a message.

public async Task Send(string? subtitle, string? title, string message, string deviceId)
{
	var payload =
	"{ \"aps\": { " +
			"\"alert\": { " +
				"\"title\": \"" + title + "\", " +
				"\"subtitle\": \"" + subtitle + "\", " +
				"\"body\": \"" + message + "\"" +
			"} " +
		"} " +
	"}";

	try
	{
		Debug.WriteLine($"apple api request: {payload}");

		using var client = new TcpClient(_hostname, _port);

		var sslStream = new SslStream(client.GetStream(), false, new RemoteCertificateValidationCallback(ValidateServerCertificate));

		var certificatesCollection = new X509Certificate2Collection(_certificate);

		await sslStream.AuthenticateAsClientAsync(_hostname, certificatesCollection, SslProtocols.Tls12 | SslProtocols.Tls13, true);

		var array = DataToBytes(deviceId, payload);
		sslStream.Write(array);
		sslStream.Flush();

		client.Close();
	}
	catch(Exception exception)
	{
		Debug.WriteLine(exception);
	}
}

The certificate that is used in this code was read beforehand:

var clientCertificate = new X509Certificate2(iOSApp.Certificate, iOSApp.CertificatePassword);

We are using a p12 certificate file with a private password.

Again, it works great for an App in Production but not at all for an App in Development. I made use that I use a new p12 developer certificate for the new App when I tested. I also tested with a p12 production certificate. I get the same error.

Any help is appreciated.