It was the crazy couple of month ago when all payment gateway did enforce the security standard to TLS 1.2.
There were systems which were built in .Net Framework 4.0 and enforcing to enable TLS 1.2 was a bit tricky there
Here is the one-liner that fixes TLS 1.2 Issue. Add this in Global.asax and you are good to go!(Ref)
ServicePointManager.SecurityProtocol = (SecurityProtocolType) 3072;
We are all set 🙂