C# – Could not create SSL/TLS secure channel. SecureChannelFailure

I'm getting an SSL error when making a SOAP call with an SSL certificate:

The request was aborted: Could not create SSL/TLS secure channel.

The weird thing is that if I load the certificate in Firefox and visit the endpoint or make a call to the API without sending any data, I don't get any error message and it connects successfully. The company exposing the API has also mentioned that the certificate is kosher.

The certificate I'm loading has full privileges to "Everyone". I've tried every solution I've seen on the internet but still getting the error.

Here is my code that creates the request:

 ServicePointManager.Expect100Continue = true;
 ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
 var request = (HttpWebRequest)WebRequest.Create(plugin.EndPoint);
 request.ContentType = "text/xml; charset=utf-8";
 request.Method = "POST";

The code to get the certificate (I've also tried with a pfx):

var cert = new 
 509Certificate2(@"C:\clientcert.p12", "FakePassword");
request.ClientCertificates.Add(cert);

and the code for the request:

  byte[] byteArray = Encoding.UTF8.GetBytes(xml);
    request.ContentLength = byteArray.Length;
    using (var dataStream = request.GetRequestStream())
                {
                    dataStream.Write(byteArray, 0, byteArray.Length);
                    dataStream.Close();


                    using (WebResponse response = request.GetResponse())
                    {
                        using (var responseStream = response.GetResponseStream())
                        {
                            StreamReader reader = 
new StreamReader(responseStream ?? throw new InvalidOperationException());
                            return reader.ReadToEnd();
                        }
                    }

                }

Edit:

Here is the trace output from running the request:

System.Net Information: 0 : [11844]
InitializeSecurityContext(In-Buffers count=2, Out-Buffer length=0,
returned code=CertUnknown). System.Net Error: 0 : [11844] Exception in
HttpWebRequest#63832831:: – The request was aborted: Could not create
SSL/TLS secure channel.. System.Net Error: 0 : [11844] Exception in
HttpWebRequest#63832831::EndGetRequestStream – The request was
aborted: Could not create SSL/TLS secure channel..

I also changed the SecurityProtocol:

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

Second Edit: I can get it to work in SoapUI but not in the .NET application by just loading the SSL certificate from the file system in SOAP UI.