An unexpected error occurred on a send

THE UNDERLYING CONNECTION WAS CLOSED: AN UNEXPECTED ERROR OCCURRED ON A SEND

.net framework 4.5

Server TLS: TLS1.2

My code: Please guide me how to resolve the issue.

HttpWebRequest req = (HttpWebRequest)(HttpWebRequest.Create(url));

req.Method = "POST";

req.KeepAlive = false;

req.ProtocolVersion = HttpVersion.Version10;

req.ContentType = "application/xml ";

req.Headers.Set(HttpRequestHeader.Authorization, "Basic API KEY");

ServicePointManager.SecurityProtocol =SecurityProtocolType.Tls12;

req.Timeout = 30000;

req.ReadWriteTimeout = 30000;

XmlDocument XDAuth = new XmlDocument();

XmlNode docNode = XDAuth.CreateXmlDeclaration("1.0", "UTF-8", "yes");

XDAuth.AppendChild(docNode);

XmlNode Root = XDAuth.AppendChild(XDAuth.CreateElement("xml"));

XmlNode otpnode = Root.AppendChild(XDAuth.CreateElement("Name"));

otpnode.InnerText = device;

otpnode = Root.AppendChild(XDAuth.CreateElement("ROOM"));

otpnode.InnerText = "PSL";

string content = XDAuth.OuterXml;

req.ContentLength = content.Length;

Stream wri = null;

using (wri = req.GetRequestStream())

{

byte[] array = Encoding.UTF8.GetBytes(content);

wri.Write(array, 0, array.Length);

}

wri.Flush();

wri.Close();

string resultData = "";

using (HttpWebResponse HttpWResp = (HttpWebResponse)req.GetResponse())

{

int resCode = (int)HttpWResp.StatusCode;

using (Stream stream = HttpWResp.GetResponseStream())

{

using (StreamReader reader = new StreamReader(stream))

{

resultData = reader.ReadToEnd();

resultData = resultData.Trim();

}

string decodedresult = resultData;