SIM800h cannot connect to Azure IoT Hub using Sas

I have a SIM800h gsm unit, and a Arduino MKRZERO. I hope to connect this to Azure IoT hub, using Shared Access Signatures. The device is registered in the it hub, and I can grab a access signature from the azure iot vscode extension. Originally, I was using the MKRGSM1400, and libraries associated with that, however, we found these were unreliable and unable to connect constantly. Therefore, we moved to the MKRZERO and sim800

Now, I am using the TinyGSM and PubSubClient libraries for this. My Sim800 has R13.08 hardware. The code I am using is based on the tinyGSM examples, and looks like:

// Select your modem:

// Set serial for debug console (to the Serial Monitor, default speed 115200)
#define SerialMon Serial

// Set serial for AT commands (to the module)
// Use Hardware Serial on Mega, Leonardo, Micro
#ifndef __AVR_ATmega328P__
#define SerialAT Serial1

// or Software Serial on Uno, Nano
#include <SoftwareSerial.h>
SoftwareSerial SerialAT(2, 3);  // RX, TX

// See all AT commands, if wanted

// Define the serial console for debug prints, if needed
#define TINY_GSM_DEBUG SerialMon

// Range to attempt to autobaud
// NOTE:  DO NOT AUTOBAUD in production code.  Once you've established
// communication, set a fixed baud rate using modem.setBaud(#).
#define GSM_AUTOBAUD_MIN 9600
#define GSM_AUTOBAUD_MAX 115200

// Add a reception delay, if needed.
// This may be needed for a fast processor at a slow baud rate.
// #define TINY_GSM_YIELD() { delay(2); }

// Define how you're planning to connect to the internet.
// This is only needed for this example, not in other code.
#define TINY_GSM_USE_GPRS true
#define TINY_GSM_USE_WIFI false

// set GSM PIN, if any
#define GSM_PIN ""

// Your GPRS credentials, if any
const char apn[]      = "";
const char gprsUser[] = "wap";
const char gprsPass[] = "wap";

// Your WiFi connection credentials, if applicable
const char wifiSSID[] = "YourSSID";
const char wifiPass[] = "YourWiFiPass";

// MQTT details
const char* broker = "";
const char* mqtt_user = "";
const char* mqtt_pass = "SharedAccessSignature sr=YYYYYYYYYY";

const char* topicInit      = "GsmClientTest/init";

const char* topicLed =  "devices/Arduino/messages/devicebound/#";
const char* topicLedStatus       = "devices/Arduino/messages/events/";

#include <TinyGsmClient.h>
#include <PubSubClient.h>

#include <StreamDebugger.h>
StreamDebugger debugger(SerialAT, SerialMon);
TinyGsm        modem(debugger);
TinyGsm        modem(SerialAT);
TinyGsmClient  client(modem);
PubSubClient  mqtt(client);

#define LED_PIN 13
int ledStatus = LOW;

uint32_t lastReconnectAttempt = 0;

void mqttCallback(char* topic, byte* payload, unsigned int len) {
  SerialMon.print("Message arrived [");
  SerialMon.print("]: ");
  SerialMon.write(payload, len);

  // Only proceed if incoming message's topic matches
  if (String(topic) == topicLed) {
    ledStatus = !ledStatus;
    digitalWrite(LED_PIN, ledStatus);
    mqtt.publish(topicLedStatus, ledStatus ? "1" : "0");

boolean mqttConnect() {
  SerialMon.print("Connecting to ");

  // Connect to MQTT Broker
 // boolean status = mqtt.connect("GsmClientTest");

  // Or, if you want to authenticate MQTT:
  boolean status = mqtt.connect("Arduino", mqtt_user, mqtt_pass);

  if (status == false) {
    SerialMon.println(" fail");
    return false;
  SerialMon.println(" success");
  mqtt.publish(topicLedStatus, "GsmClientTest started");
  return mqtt.connected();

void setup() {
  // Set console baud rate

  pinMode(LED_PIN, OUTPUT);

  // !!!!!!!!!!!
  // Set your reset, enable, power pins here
  // !!!!!!!!!!!


  // Set GSM module baud rate
  // SerialAT.begin(9600);

  // Restart takes quite some time
  // To skip it, call init() instead of restart()
  SerialMon.println("Initializing modem...");
  // modem.init();

  String modemInfo = modem.getModemInfo();
  SerialMon.print("Modem Info: ");

  // Unlock your SIM card with a PIN if needed
  if (GSM_PIN && modem.getSimStatus() != 3) { modem.simUnlock(GSM_PIN); }

  // The XBee must run the gprsConnect function BEFORE waiting for network!
  modem.gprsConnect(apn, gprsUser, gprsPass);

  SerialMon.print("Waiting for network...");
  if (!modem.waitForNetwork()) {
    SerialMon.println(" fail");
  SerialMon.println(" success");

  if (modem.isNetworkConnected()) { SerialMon.println("Network connected"); }

  // GPRS connection parameters are usually set after network registration
  SerialMon.print(F("Connecting to "));
  if (!modem.gprsConnect(apn, gprsUser, gprsPass)) {
    SerialMon.println(" fail");
  SerialMon.println(" success");

  if (modem.isGprsConnected()) { SerialMon.println("GPRS connected"); }

  // MQTT Broker setup
  mqtt.setServer(broker, 8883);

void loop() {
  // Make sure we're still registered on the network
  if (!modem.isNetworkConnected()) {
    SerialMon.println("Network disconnected");
    if (!modem.waitForNetwork(180000L, true)) {
      SerialMon.println(" fail");
    if (modem.isNetworkConnected()) {
      SerialMon.println("Network re-connected");

    // and make sure GPRS/EPS is still connected
    if (!modem.isGprsConnected()) {
      SerialMon.println("GPRS disconnected!");
      SerialMon.print(F("Connecting to "));
      if (!modem.gprsConnect(apn, gprsUser, gprsPass)) {
        SerialMon.println(" fail");
      if (modem.isGprsConnected()) { SerialMon.println("GPRS reconnected"); }

  if (!mqtt.connected()) {
    SerialMon.println("=== MQTT NOT CONNECTED ===");
    // Reconnect every 10 seconds
    uint32_t t = millis();
    if (t - lastReconnectAttempt > 10000L) {
      lastReconnectAttempt = t;
      if (mqttConnect()) { lastReconnectAttempt = 0; }

  if (millis && modem.isNetworkConnected() && mqtt.connected()) {
    // Publish a message every second
    if (millis() % 1000 == 0) {
      SerialMon.println("sending message!");
      mqtt.publish(topicLedStatus, "hello :)");


I believe I have set the correct port, broker and using mqtt.connect correctly. However, when I upload this code to my arduino, I get stuck in the in the loop entering mqttConnect(), with the output:

Connecting to[651883] ### Closed:  0

The arduino does manage to connect to the network successfully and can connect to other MQTT brokers and send/receive messages to/from them. I do not attempt to handle any SSL certificates, though I don’t believe this should be an issue, as I am using symmetric key/ SAS security for connecting.

Does anyone have any suggestions on what may be causing this behaviour? I can provide more/less code, or run more debugging if asked. thanks.

If I connect the sim800H directly to my PC, and use PuTTY to send AT commands to it, I can see that SSL related AT commands do in fact work (ie: AT+SSLOPT=1,1 does not cause an error), so I think Im on a firmware that can handle SSL - if that helps