Can't connect to broker using CA Cert ClientCert and Key

I try to connect to my MQTT broker with TLS. I have these 3 certificates as follows
-CA Cert (CRT)
-Client Cert (PEM)
-Client Key (PEM)

These certificates are valid and work if I use them with a software like MQTTX.

Now I am trying to use MqttNet (3.1.2) to connect to broker. I have read the documentation and accordingly tried by converting clientCert and Key to pfx file using following command.

`openssl pkcs12 -export -out certificate.pfx -inkey clientkey.pem -in clientCert.crt`

My MQTTnet code :
`// Creates a new client
MqttClientOptionsBuilder builder = new MqttClientOptionsBuilder()
.WithClientId("Test")
.WithTcpServer("mqtt.cmec.iotexpress.io", 8889);

var clientCert_mos = new X509Certificate2(@"C:\Temp\certificate.pfx");
var caCrt_mos = new X509Certificate2(@"C:\Temp\ca-chain2.pem");

builder
.WithCredentials("User", "Password")
.WithTls(
new MqttClientOptionsBuilderTlsParameters()
{
UseTls = true,
SslProtocol = System.Security.Authentication.SslProtocols.Tls12,
AllowUntrustedCertificates = true,

CertificateValidationHandler = (certContext) => {
X509Chain chain = new X509Chain();
chain.ChainPolicy.RevocationMode = X509RevocationMode.NoCheck;
//chain.ChainPolicy.RevocationFlag = X509RevocationFlag.ExcludeRoot;
//chain.ChainPolicy.VerificationFlags = X509VerificationFlags.NoFlag;
//chain.ChainPolicy.VerificationTime = DateTime.Now;
//chain.ChainPolicy.UrlRetrievalTimeout = new TimeSpan(0, 0, 0);
chain.ChainPolicy.CustomTrustStore.Add(caCrt_mos);
chain.ChainPolicy.TrustMode = X509ChainTrustMode.CustomRootTrust;

// convert provided X509Certificate to X509Certificate2
var x5092 = new X509Certificate2(certContext.Certificate);
return chain.Build(x5092);
},

Certificates = new List()
{
caCrt_mos ,
}
})
.Build(); `

I can't connect.

I did try to connet to Test.mosquitto.org on port 8884 (Encrypted and client certificate required (Ca, Cert, Key), same as mine) and I can establish the connection...

Any idea what I'm doing wrong?

Thanks,