Unable to connect to Azure Cosmos DB using the MongoDB Driver .Net C#

Per my understanding, you could use SslSettings.ServerCertificateValidationCallback for getting more details about the server certificate validation and check the policy errors to narrow this issue. You could construct your MongoClientSettings as follows for building your MongoClient:

MongoClientSettings settings = MongoClientSettings.FromUrl(new MongoUrl("{connection-string-of-your-cosmosdb}"));
settings.SslSettings = new SslSettings()
{
    EnabledSslProtocols = SslProtocols.Tls12,
    ServerCertificateValidationCallback = delegate (object s, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
    {
        foreach (var element in chain.ChainElements)
        {
            // Gets the error status of the current X.509 certificate in a chain.
            foreach (var status in element.ChainElementStatus)
            {
                Console.WriteLine($"certificate subject: {element.Certificate.Subject},ChainStatus: {status.StatusInformation}");
            }
        }
        return true; //just for dev, it would bypass certificate errors
    }
};
var mongoClient = new MongoClient(settings);