In .NET 4.0 default transport-level security standard is TLS 1.1. The solution is to upgrade the site to .NET 4.6 or above. In this framework version, TLS 1.2 is a default cryptographic standard.
But what if you can't upgrade your application to the latest .NET framework, then please use the below code. The following code will make TLS 1.2 default, make sure to execute it before making a connection to secured resource:
System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12;
