APNs Certificate Based Push Notificatoin

Question

I'm trying to send push notifications via APNs via certificate based authentication (token based is not an option in my scenario). When I run the code, I get the following exceptions on SendAsync:

1. HttpRequestException: Error while copying content to a stream.
2. IOException: The write operation failed, see inner exception.
3. WinHttpException: Error 12152 calling WinHttpWriteData, 'The server returned an invalid or unrecognized response'.

If I comment out the "handler.ClientCertificate.Add(_certificate)", no exceptions but I get a response status code of 403 and reason of "MissingProviderToken". So to mean this implies that it is able to establish a http2 connection.

I've been at the last two days so any help will be much appreciated!

var handler = new WinHttpHandler();
        handler.ClientCertificateOption = ClientCertificateOption.Manual;
        handler.ClientCertificates.Add(_certificate);
        handler.AutomaticDecompression = DecompressionMethods.GZip |
                                     DecompressionMethods.Deflate;          
        HttpClient client = new HttpClient(handler);

        var pushToken = "Base64EndocedDeviceTokenId";
        var pushTokenByteArray = Convert.FromBase64String(pushToken);
        var pushTokenHexString = BitConverter.ToString(pushTokenByteArray).Replace("-", "");
        Dictionary<string, object> body;
        body = new Dictionary<string, object>
 {
    { "aps", new Dictionary<string, object>() { { "alert" , "Hello" } } },
};

        var bodyStr = JsonConvert.SerializeObject(body);

        var requestMessage = new HttpRequestMessage
        {
            Method = HttpMethod.Post,
            Content = new StringContent(bodyStr, Encoding.UTF8, "application/json"),
            RequestUri = new Uri($"https://{_host}/3/device/{pushTokenHexString}"),
            Version = new Version(2, 0)
        };
        requestMessage.Headers.Add("apns-push-type", "alert");
        requestMessage.Headers.Add("apns-topic", "my.bundle.id");
        requestMessage.Headers.Add("apns-priority", "10");
        var response1 = client.SendAsync(requestMessage).Result;
        var responseBody = response1.Content.ReadAsStringAsync().Result;