When I run the program, I can see the "GSM networks scanner" message appear on my serial monitor, but it does not display anything else after that. Also, the NET and STATUS LEDs turn off after some few seconds when the program starts running. I already made the minor adjustments necessary for the Arduino Mega, and the power supply is temporarily from the USB port of my laptop connected to the Arduino Mega. I am using a SIM card that has been activated.

This is the program that I am using:

*/

// libraries
#include <GSM.h>

// PIN Number
#define PINNUMBER ""

// initialize the library instance
GSM gsmAccess; // include a 'true' parameter to enable debugging
GSMScanner scannerNetworks;
GSMModem modemTest;

// Save data variables
String IMEI = "";

// serial monitor result messages
String errortext = "ERROR";

void setup() {
 // initialize serial communications and wait for port to open:
 Serial.begin(9600);

 Serial.println("GSM networks scanner");
 scannerNetworks.begin();

 // connection state
 boolean notConnected = true;

 // Start GSM shield
 // If your SIM has PIN, pass it as a parameter of begin() in quotes
 while (notConnected) {
 if (gsmAccess.begin(PINNUMBER) == GSM_READY) {
 notConnected = false;
 } else {
 Serial.println("Not connected");
 delay(1000);
 }
 }

 // get modem parameters
 // IMEI, modem unique identifier
 Serial.print("Modem IMEI: ");
 IMEI = modemTest.getIMEI();
 IMEI.replace("\n", "");
 if (IMEI != NULL) {
 Serial.println(IMEI);
 }
}

void loop() {
 // scan for existing networks, displays a list of networks
 Serial.println("Scanning available networks. May take some seconds.");
 Serial.println(scannerNetworks.readNetworks());

 // currently connected carrier
 Serial.print("Current carrier: ");
 Serial.println(scannerNetworks.getCurrentCarrier());

 // returns strength and ber
 // signal strength in 0-31 scale. 31 means power > 51dBm
 // BER is the Bit Error Rate. 0-7 scale. 99=not detectable
 Serial.print("Signal Strength: ");
 Serial.print(scannerNetworks.getSignalStrength());
 Serial.println(" [0-31]");

}

It would be great if you could be more specific when providing helpful information.

P.S. when I run the program without inserting a SIM card, the NET and STATUS LEDs keep on working (NET is blinking, while STATUS remains ON)

I tried this program as well https://www.arduino.cc/en/Guide/ArduinoGSMShield#toc9
and it has the same problem