Avigdor Greetings!
Can you share more details about the error?
Did you check this TLS/SSL configuration?
If the SAS token is correct, then it's possible that there is an issue with the MQTT connection. You can try enabling logging in the SDK to see if there are any error messages that can help you diagnose the issue.
Regarding your question about server authentication, if you are using TLS without server authentication, you don't need a server certificate. If you want to use server authentication, you can obtain the server certificate. You can find more information on how to obtain the server certificate in the Communicate with an IoT hub using the MQTT protocol.
Do let me know if that helps or have any further queries.