We are extending the source code of a module that targets .NET Framework version 4.5.  Our intent is to use System.Net.Http.HttpClient to send relevant information to a payment API and obtain the response.  However, our diagnostics have shown the following error:  

Exception in HttpClientHandler#31129591::SendAsync - The underlying connection was closed: An unexpected error occurred on a send..

I understand that .NET Framework 4.5 uses an older Transport Layer Security (TLS) than more modern APIs utilize.  I'm thinking this is a root cause but am unsure whether there's something else amiss.  The code that calls this API is as follows:

HttpClientHandler handler = new HttpClientHandler

            {

                AllowAutoRedirect = true

            };

            TransactionResult_ProPay result = new TransactionResult_ProPay();

            using (HttpClient client = new HttpClient(new LoggingHandler(new HttpClientHandler())))

            using (HttpClient client = new HttpClient(handler))

            {

                HttpRequestMessage request = new HttpRequestMessage();

                HttpResponseMessage response = new HttpResponseMessage();

                response.EnsureSuccessStatusCode();

                client.BaseAddress = new Uri("https://xmltestapi.propay.com/ProtectPay/HostedTransactions/");

                ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls;

                client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

                string responseBody;

                CreateHostedTransactionRequest transactionRequest = new CreateHostedTransactionRequest();

                CreateHostedTransactionResponse transactionResponse = new CreateHostedTransactionResponse();

                HttpContent content = new StringContent(transactionRequest.ToString());

                try

                {

                    request.Headers.Add("Authorization", credentials);

                    request.Headers.Add("accept", "application/json");

                    request.Method = HttpMethod.Put;

                    response = await client.PutAsync("https://xmltestapi.propay.com/ProtectPay/HostedTransactions/", content);

                    responseBody = await response.Content.ReadAsStringAsync();

                    objEventLog.AddLog("Transaction completed", portalSettings, userId,

                        response.ToString(), EventLogController.EventLogType.ADMIN_ALERT);

                    Console.WriteLine(responseBody);

                }

                catch (HttpRequestException ex)

                {

                    result.Succeeded = false;

                    result.ResultMessage = ex.Message;

                    objEventLog.AddLog("Transaction Request Error", portalSettings, userId,

                        ex.Message,

                        EventLogController.EventLogType.ADMIN_ALERT);

                    Console.WriteLine("\nException Caught!");

                    Console.WriteLine("Message :{0} ", ex.Message);

                }

                return response.Content.ToString();

It seems like using System.Net.Http.HttpClient may not be the best fit due to the mismatch in TLS versions.  However, before I embark on another approach (calling the API from a .NET Core web service and sending the responses back to the DNN database), I wanted to obtain input as to whether there are other issues preventing a successful connection.  Thanks much for your help and guidance.