I found the subscribing topics in this code could cause Arduino hanging after few hours running. Recommend not use this for now.

// Include the library to read sensor data
#include <DHT11.h>

// Include mqtt client library
#include <ArduinoMqttClient.h>
#if defined(ARDUINO_SAMD_MKRWIFI1010) || defined(ARDUINO_SAMD_NANO_33_IOT) || defined(ARDUINO_AVR_UNO_WIFI_REV2)
#include <WiFiNINA.h>
#elif defined(ARDUINO_SAMD_MKR1000)
#include <WiFi101.h>
#elif defined(ARDUINO_ARCH_ESP8266)
#include <ESP8266WiFi.h>
#elif defined(ARDUINO_PORTENTA_H7_M7) || defined(ARDUINO_NICLA_VISION) || defined(ARDUINO_ARCH_ESP32) || defined(ARDUINO_GIGA) || defined(ARDUINO_OPTA)
#include <WiFi.h>
#elif defined(ARDUINO_PORTENTA_C33)
#include <WiFiC3.h>
#elif defined(ARDUINO_UNOR4_WIFI)
#include <WiFiS3.h>
#endif

// define pin for sensor data read
DHT11 dht11(2);

// Don't change
char mqtt_user[] = "energy";
char mqtt_pass[] = "password";

// Don't change
const char broker[] = "9.tcp.ngrok.io";  //IP address of the EMQX broker.
int port = 24004;

// The SSID and password we are going to use in NAVY YARD, if you want to test, use your own wifi ssid and password
char ssid[] = "NewLabMember 2.4GHz Only";  // your network SSID (name)
char pass[] = "!Welcome2NewLab!";      // your network password

// Set up topics, you want to subscribe and publish, you can name whatever you want
// For instance:
const char subscribe_topic[] = "/energy_solar_project/group5";
const char publish_topic[] = "/energy_solar_project/group5";

WiFiClient wifiClient;
MqttClient mqttClient(wifiClient);

void setup() {
  // Create serial connection and wait for it to become available.
  Serial.begin(9600);

  // Connect to WiFi access point.
  Serial.print("Connecting to ");
  Serial.println(ssid);

  delay(1000);

  WiFi.begin(ssid, pass);
  delay(2000);

  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println();

  Serial.println("You're connected to the network");
  Serial.println();

  // You can provide a username and password for authentication
  mqttClient.setUsernamePassword(mqtt_user, mqtt_pass);

  Serial.print("Attempting to connect to the MQTT broker.");

  if (!mqttClient.connect(broker, port)) {
    Serial.print("MQTT connection failed! Error code = ");
    Serial.println(mqttClient.connectError());

    while (1)
      ;
  }

  Serial.println("You're connected to the MQTT broker!");

  Serial.print("Subscribing to topic: ");
  Serial.println(subscribe_topic);

  // subscribe to a topic
  mqttClient.subscribe(subscribe_topic);

  // topics can be unsubscribed using:
  // mqttClient.unsubscribe(topic);

  Serial.print("Waiting for messages on topic: ");
  Serial.println(subscribe_topic);
}

void loop() {
  int temperature = 0;
  int humidity = 0;

  // Attempt to read the temperature and humidity values from the DHT11 sensor.
  int result = dht11.readTemperatureHumidity(temperature, humidity);

  // Check the results of the readings.
  // If the reading is successful, print the temperature and humidity values.
  // If there are errors, print the appropriate error messages.
  if (result == 0) {
    Serial.print("Temperature: ");
    Serial.print(temperature);
    Serial.print(" °C\\tHumidity: ");
    Serial.print(humidity);
    Serial.println(" %");

    // send message, the Print interface can be used to set the message contents
    // notice the way we structure data here, 
    // we are making a JSON like string for the server to parse and store into the database
    // the data below will look like this {"temperature":24,"humidity":18}
    
    mqttClient.beginMessage(publish_topic);
    mqttClient.print("{\\"temperature\\":");
    mqttClient.print(temperature);
    mqttClient.print(",\\"humidity\\":");
    mqttClient.print(humidity);
    mqttClient.print("}");
    mqttClient.endMessage();

    // if you only have one data to send, you can comment the code above and use this instead
    // The data will look like {"temperature":24}
    // mqttClient.beginMessage(publish_topic);
    // mqttClient.print("{\\"temperature\\":");
    // mqttClient.print(temperature);
    // mqttClient.print("}");
    // mqttClient.endMessage();

  } else {
    // Print error message based on the error code.
    Serial.println(DHT11::getErrorString(result));
  }

  // if getting new data from subscribing topic, print them out in Serial Monitor
  int messageSize = mqttClient.parseMessage();
  if (messageSize) {
    // we received a message, print out the topic and contents
    Serial.print("Received a message with topic '");
    Serial.print(mqttClient.messageTopic());
    Serial.print("', length ");
    Serial.print(messageSize);
    Serial.println(" bytes:");

    // use the Stream interface to print the contents
    while (mqttClient.available()) {
      Serial.print((char)mqttClient.read());
    }
    Serial.println();
  }

  delay(3000);
}