How to connect Emqx using cellular Netwrok

[Nikitanagar]

I am trying to connect my ESP32 with EMQX over a cellular network using a SIM7600 module. The same code works fine over WiFi, but it fails when using GPRS/LTE.
Successfully connected the SIM7600 to the network (I can get an IP address).

Here is my code:

`#define TINY_GSM_MODEM_SIM7600 // SIM7600 AT instruction is compatible with A7670
#define TINY_GSM_USE_GPRS true
#include <PubSubClient.h>
#include "credentials.h"
#include <ArduinoJson.h>
#include <TinyGsmClient.h>
#include <TinyGPSPlus.h>

#define RXD2 27
#define TXD2 26

#define SerialAT Serial1
#define SerialGPS Serial2

#define GNSS_enable 14 //Set as LOW
#define GSM_Power_Sequence 33

const char apn[] = "";
const char gprsUser[] = "";
const char gprsPass[] = "";

#ifdef DUMP_AT_COMMANDS
#include <StreamDebugger.h>
StreamDebugger debugger(SerialAT, Serial);
TinyGsm modem(debugger);
#else
TinyGsm modem(SerialAT);
#endif
TinyGsmClient client(modem);

PubSubClient mqtt(client);

// Function Declarations
void connectToMQTT();
void mqttCallback(char *topic, byte *payload, unsigned int length);

void setup()
{
Serial.begin(115200);
SerialAT.begin(115200, SERIAL_8N1, RXD2, TXD2);
SerialGPS.begin(115200, SERIAL_8N1, 17, 16);
delay(5000);

pinMode(GNSS_enable, OUTPUT);
pinMode(GSM_Power_Sequence, OUTPUT);

if (!modem.init()) {
DBG("Failed to restart modem, delaying 10s and retrying");
return;
}

digitalWrite(GNSS_enable, LOW);
digitalWrite(GSM_Power_Sequence, HIGH); //GSM Power Up Sequence
delay(1500);
digitalWrite(GSM_Power_Sequence, LOW);
delay(500);

String name = modem.getModemName();
DBG("Modem Name:", name);

String modemInfo = modem.getModemInfo();
DBG("Modem Info:", modemInfo);

Serial.println("Waiting for network...");
if (!modem.waitForNetwork()) {
Serial.println(" fail");
delay(10000);
return;
}
Serial.println(" success");

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

// GPRS connection parameters are usually set after network registration
Serial.print(F("Connecting to "));
Serial.print(apn);
if (!modem.gprsConnect(apn, gprsUser, gprsPass)) {
Serial.println(" fail");
delay(10000);
return;
}
Serial.println(" success");

if (modem.isGprsConnected()) {
Serial.println("LTE module connected");
}

mqtt.setServer(mqtt_broker, mqtt_port);
mqtt.setCallback(mqttCallback);

}

void connectToMQTT()
{
while (!mqtt.connected()){
Serial.print("Attempting MQTT connection...");

boolean status =  mqtt.connect(clientId, mqtt_username, mqtt_password);

if(status == false){
  Serial.println(" fail");
  Serial.print("failed, rc=");
  Serial.print(mqtt.state());
  Serial.println(" try again in 5 seconds");
  delay(5000);  // Will attempt connection again in 5 seconds
}
else
{
  Serial.print("success  ");

}

}
}

void mqttCallback(char *topic, byte *payload, unsigned int length)
{
Serial.print("Message received on topic: ");
Serial.println(topic);
Serial.print("Message: ");
for (unsigned int i = 0; i < length; i++)
{
Serial.print((char)payload[i]);
}
Serial.println("\n-----------------------");
}

void loop()
{
if (!mqtt.connected())
{
connectToMQTT();
}
mqtt.loop();
delay(100);
}`

Error: The MQTT connection fails with rc=-2 or rc=-4.

Problem:
Works fine over WiFi.
Over GSM, the connection fails.
The device gets an IP address, but the MQTT handshake does not complete.

Questions:
Does EMQX require any special settings for cellular MQTT connections?
Is there a way to debug the handshake process?
Any recommended APN settings or additional AT commands for SIM7600?

Any help would be appreciated. Thanks!