SIM800l no data available from serial port

[nanarod]

I was testing the sim800l module with a code to read SMS from the following blog.
below which is from Controlar dispositivos electrónicos a través de mensajes de texto – SIM800L y Arduino – El Taller De TD

At first, it worked properly but suddenly it stopped receiving data. It seems like the module is connected to the network because the light blinks every 3 seconds but the sim either doesn't receive any data when is in the module or it cannot deliver it to the Arduino. If I use Serial.read() (in my code SIM800L.read()) it returns -1 which means that there's no data to read.

The first time I used the module I was working with an Arduino UNO and it was working perfectly but I had to change to an Arduino MEGA 2560 and then started to happen what I described before. I decided to go back to Arduino UNO but the problem persists. I have checked if is an alimentation problem or that I burn the sim800l but that is not the case, I even buy a new module thinking maybe I had damaged the RX pin but the new one doesn't work either.

This is the code I'm using:

#include <SoftwareSerial.h> //Incluimos la libreria Software Serial

//Creado por youtube/ElTallerDeTD
//eltallerdetd@gmail.com

SoftwareSerial SIM800L(8,9); //Instanciamos el objeto SIM800L y le pasamos los parametros de los pines TX y RX
String valor;  //Declaramos la variable de tipo String valor.
int num;

void setup() {
  pinMode(13, OUTPUT); //Declarar el pin 13 como salida.
  Serial.begin(115200); //Inicializamos la primera comunicacion Serial.
  SIM800L.begin(115200); //Inicializamos la segunda comunicacion Serial.
  SIM800L.println("AT+CMGF=1"); //Vamos utilizar los SMS.
  delay(100); //Delay de 0.1 sec
  SIM800L.println("AT+CNMI=1,2,0,0,0"); //Configurar el SIM800L p/ que muestre msm por com. serie.
}

void loop() {
  //Lograr que nos muestre lo que nos llega de mensaje por el monitor serial.
  valor = SIM800L.read(); //Guardar en la var valor el sms que recibe el Arduino
    Serial.println("Nuevo SMS: "+ valor); //Imprime ese SMS en el monitor Serial
  
  if(SIM800L.available()>0){
    valor = SIM800L.readString(); //Guardar en la var valor el sms que recibe el Arduino
    Serial.println("Nuevo SMS: "+ valor); //Imprime ese SMS en el monitor Serial
  }
  
  if(valor.indexOf("ON")>=0){  //Si la var valor tiene la palabra ON hace esto:
    digitalWrite(13, HIGH);    //Se enciende el pin 13.
    Serial.println("El Led se encendio");  //Immprime el mensaje
    delay(15000);
  }else{
    digitalWrite(13, LOW);    //Se apaga el pin 13.
    Serial.println("El Led se apago");  //Immprime el mensaje
    delay(15000);
  
  }
  
}

NOTE: I have used many other codes and still have the same problem, but I think this is the easiest that I found.