Possibility of device to cloud communication failure even when the device shows as running

This error occurs when the code has been successfully uploaded to the device and we are trying to test the device and its communication with the cloud. Since it is using MQTT, we need to ensure that the firewall and router are not blocking the communication.

Author: neerajks77

articles/iot-hub/iot-hub-arduino-iot-devkit-az3166-get-started.md

@@ -317,6 +317,17 @@ The sample application is running successfully when you see the following result
 
 ![Serial monitor output](media/iot-hub-arduino-devkit-az3166-get-started/getting-started/result-serial-output.png)
 
+> [!NOTE]
+>In case you receive the below errors during testing, notice that the LED is not blinking, and the Azure Portal does not show the incoming data from the device, but the device OLED screen shows as Running... go to the device inside the IoT Hub within Azure Portal and send message to the device. If you see the response in the serial monitor within VS Code, there is a possibility that the direct communication from the device is getting blocked at the router level. You can check for the firewall or the router rules configured for the connecting devices. Check if the outbound port 1833 is open as well.
+>ERROR: mqtt_client.c (ln 454): Error: failure opening connection to endpoint
+>INFO: >>>Connection status: disconnected
+>ERROR: tlsio_mbedtls.c (ln 604): Underlying IO open failed
+>ERROR: mqtt_client.c (ln 1042): Error: io_open failed
+>ERROR: iothubtransport_mqtt_common.c (ln 2283): failure connecting to address atcsliothub.azure-devices.net.
+>INFO: >>>Re-connect.
+>INFO: IoThub Version: 1.3.6
+
+
 ### View the telemetry received by Azure IoT Hub
 
 You can use [Azure IoT Tools](https://marketplace.visualstudio.com/items?itemName=vsciot-vscode.azure-iot-tools) to monitor device-to-cloud (D2C) messages in IoT Hub.