I'm using a GSM Module with Arduino along with Node MCU. I have a supply of 9V, 2A in the DC jack of the Arduino and the GSM Shield. The Node MCU is powered from the 3.3V of the Arduino. I can receive messages fine in the GSM when the USB port of the Arduino (including the power supply I mentioned earlier) is connected to the laptop through and monitored using the serial monitor. However if I remove the USB port, the GSM cannot receive the messages anymore but when I press the reset button of the Arduino, it will start receiving messages again. Why does it only works when I reset the Arduino first after powering it up?

#include <SoftwareSerial.h>
#include <Wire.h>

SoftwareSerial gsmSerial(2, 3);

char incomingByte;
String inputString;

// Pin configurations.

const int outlet1Pin = 4;
const int outlet2Pin = 5;
const int outlet3Pin = 6;

const int ping1Pin = 7;
const int ping2Pin = 8;
const int ping3Pin = 9;
const int ping4Pin = 10;
const int ping5Pin = 11;
const int ping6Pin = 12;
const int signalPin = 13;

const int pingWaitTime = 3000;

void setup()
{
  Serial.begin(9600);
  gsmSerial.begin(9600);

  pinMode(outlet1Pin, INPUT);
  pinMode(outlet2Pin, INPUT);
  pinMode(outlet3Pin, INPUT);

  pinMode(ping1Pin, OUTPUT);
  pinMode(ping2Pin, OUTPUT);
  pinMode(ping3Pin, OUTPUT);
  pinMode(ping4Pin, OUTPUT);
  pinMode(ping5Pin, OUTPUT);
  pinMode(ping6Pin, OUTPUT);
  pinMode(signalPin, OUTPUT);

  initGSM();
  //Serial.flush();
  //gsmSerial.flush();
  //sendSMS("Test send.");


}

void loop()
{
  if (gsmSerial.available())
  {
    digitalWrite(signalPin, HIGH);

    delay(500);

    //Serial.println("Serial available.");

    while (gsmSerial.available())
    {
      incomingByte = gsmSerial.read();
      inputString += incomingByte;
    }

    inputString.toUpperCase();



    checkCommandAction(inputString);


    if (inputString.indexOf("OK") == -1)
    {
      gsmSerial.println("AT+CMGDA=\"DEL ALL\"");
      delay(1000);
    }

    inputString = "";
    gsmSerial.flush();

  }
  digitalWrite(signalPin, LOW);

}

void sendSMS(String text)
{
  Serial.println("Sending Text...");

  gsmSerial.println("AT+CMGF=1");
  delay(1000);

  gsmSerial.print("AT+CMGS=\"+numberhere\"\r");

  delay(1000);

  gsmSerial.print(text);
  delay(1000);

  gsmSerial.print((char)26);
  delay(1000);

  gsmSerial.println();
  Serial.println("Message sent!");
  delay(1000);
}

void initGSM()
{
  gsmSerial.println("ATE0");
  while (!gsmSerial.available())
  {
    gsmSerial.println("AT");
    delay(500);
    Serial.println("Connecting...");
  }

  Serial.println("Connected!");

  gsmSerial.println("AT+CMGF=1");
  delay(500);
  gsmSerial.println("AT+CNMI=1,2,0,0,0");
  delay(500);
  gsmSerial.println("AT+CMGL=\"REC UNREAD\"");
  delay(500);
  gsmSerial.println("AT+CMGDA=\"DEL ALL\"");
  delay(500);

  Serial.println("All messages deleted!");

  Serial.println("Sending Text...");

  gsmSerial.println("AT+CMGF=1");
  delay(500);

  gsmSerial.print("AT+CMGS=\"+639776811077\"\r");

  delay(500);

  gsmSerial.print("Device Initialized!");
  delay(500);

  gsmSerial.print((char)26);
  delay(500);

  gsmSerial.println();

  Serial.println("Message sent!");
  delay(500);
}

void checkCommandAction(String command)
{

  //Serial.println("Check Command.");

  if (command.indexOf("O1ON") > -1)
  {

    digitalWrite(ping1Pin, !LOW);
    delay(pingWaitTime);
    digitalWrite(ping1Pin, !HIGH);
    Serial.println("O1ON Enter");
    sendSMS("Outlet 1 Turned ON by text.");

  }
  else if (command.indexOf("O1OFF") > -1)
  {
    digitalWrite(ping4Pin, !LOW);
    delay(pingWaitTime);
    digitalWrite(ping4Pin, !HIGH);
    Serial.println("O1OFF");
    sendSMS("Outlet 1 Turned OFF by text.");
  }
  else if (command.indexOf("O2ON") > -1)
  {
    digitalWrite(ping2Pin, !LOW);
    delay(pingWaitTime);
    digitalWrite(ping2Pin, !HIGH);
    Serial.println("O2ON");
    sendSMS("Outlet 2 Turned ON by text.");
  }
  else if (command.indexOf("O2OFF") > -1)
  {
    digitalWrite(ping5Pin, !LOW);
    delay(pingWaitTime);
    digitalWrite(ping5Pin, !HIGH);
    Serial.println("O2OFF");
    sendSMS("Outlet 2 Turned OFF by text.");
  }
  else if (command.indexOf("O3ON") > -1)
  {
    digitalWrite(ping3Pin, !LOW);
    delay(pingWaitTime);
    digitalWrite(ping3Pin, !HIGH);
    Serial.println("O3ON");
    sendSMS("Outlet 3 Turned ON by text.");

  }
  else if (command.indexOf("O3OFF") > -1)
  {
    digitalWrite(ping6Pin, !LOW);
    delay(pingWaitTime);
    digitalWrite(ping6Pin, !HIGH);
    Serial.println("O1OFF");
    sendSMS("Outlet 3 Turned OFF by text.");
  }
  else if (command.indexOf("CHECKSTATE") > -1)
  {
    int state1 = !digitalRead(outlet1Pin);
    int state2 = !digitalRead(outlet2Pin);
    int state3 = !digitalRead(outlet3Pin);

    String toSend = "";
    toSend += "Outlet 1: ";
    toSend += String((state1) ? ("ON") : ("OFF"));
    toSend += "Outlet 2: ";
    toSend += String((state2) ? ("ON") : ("OFF"));
    toSend += "Outlet 3: ";
    toSend += String((state3) ? ("ON") : ("OFF"));

    sendSMS(toSend);
  }
}

void turnPingsOn()
{
  // The pins where the pings are connected are
  // configures as INPUT_PULL up, so we need to high
  // it by default.

  digitalWrite(ping1Pin, HIGH);
  digitalWrite(ping2Pin, HIGH);
  digitalWrite(ping3Pin, HIGH);
  digitalWrite(ping4Pin, HIGH);
  digitalWrite(ping5Pin, HIGH);
  digitalWrite(ping6Pin, HIGH);
}