To start understanding the functioning of the sim800l module I used the following example:

//Reset resistance is not soldered. If necessary, weld it yourself.

#include <M5Stack.h>
#define RX_PIN 16
#define TX_PIN 17
#define RESET_PIN 5

void header(const char *string, uint16_t color){
M5.Lcd.fillScreen(color);
M5.Lcd.setTextSize(1);
M5.Lcd.setTextColor(TFT_MAGENTA, TFT_BLUE);
M5.Lcd.fillRect(0, 0, 320, 30, TFT_BLUE);
M5.Lcd.setTextDatum(TC_DATUM);
M5.Lcd.drawString(string, 160, 3, 4);
}

void setup() {
M5.begin();

header("SIM800L AT command", TFT_BLACK);
M5.Lcd.setTextFont(2);
M5.Lcd.setTextColor(TFT_WHITE, TFT_BLACK);
M5.Lcd.drawString("Please use serial port to Test AT command.",0, 35, 2);
// Host serial communication
Serial.begin(115200);

// SIM800L serial communication
Serial2.begin(115200, SERIAL_8N1, RX_PIN, TX_PIN);
pinMode(RESET_PIN, OUTPUT);
}

void loop() {

//AT instruction write
if(Serial.available()){
Serial2.write(Serial.read());
}

//AT instruction result
if(Serial2.available()){
Serial.write(Serial2.read());
}

delay(10);

}

I sent some at command to understand if the module worked but apparently it doesn't work. Any suggestions?