Sim800l doesn't respond to AT commands

Question

My SIM800L doesn't respond to AT commands. The serial monitor is simply blank.

And the led blinks every 3 seconds which means it is connected to a network

I power it through an LM2596 buck down converter with 4.0 v (I've tried to power with voltages until 4.4v), From a 12v power supply.

Can you help me? (Please keep in mind that I'm an absolute beginner in this field. Thanks)

I used this wiring diagram from Last Minute Engineers.

And also their code:

#include <SoftwareSerial.h>
SoftwareSerial mySerial(3, 2); //SIM800L Tx & Rx is connected to Arduino #3 & #2
void setup() {
Serial.begin(9600);

mySerial.begin(9600); 

Serial.println(&quot;Initializing...&quot;); 

delay(1000);
mySerial.println(&quot;AT&quot;);

updateSerial(); 

mySerial.println(&quot;AT+CMGF=1&quot;);

updateSerial(); 

mySerial.println(&quot;AT+CNMI=1,2,0,0,0&quot;); 


updateSerial(); 

}
void loop()
{ 
    updateSerial();
}
void updateSerial() {
delay(500); 

while (Serial.available()) { 

    mySerial.write(Serial.read());
} 

while(mySerial.available()) { 

    Serial.write(mySerial.read());
}

}