serial communication problem with sim7600 with arduino mega I am just sending the AT command to test if the serial communication works, the response I am getting is a flipped ? or a square or rectangle symbol.
I have tested the communication using software provided by waveshare connected through on board usb to uart.

https://www.waveshare.com/wiki/SIM7600E-H_4G_HAT

int PowerKey = 2;
char answer;
void setup() 
{
 Serial3.begin(9600);
 Serial.begin(9600);
 pinMode(PowerKey, OUTPUT);
 Serial.println("initializing");
 digitalWrite(PowerKey, HIGH);
 delay(1000);
 digitalWrite(PowerKey, LOW);
}

void loop() 
{
  answer = send_at_cmd("AT\r\n");
  Serial.println(answer);
}


uint8_t send_at_cmd(const char* ATcommand)
{
  char response;
  // send test command AT
  Serial3.print(ATcommand);
  Serial.println(ATcommand);
  delay(1000);
  //Check for response from the sim7600E-H
  while(Serial3.available() != 0)
  {
    response = Serial3.read();
  }
  return response;
}
You have change baud rate by at+iprex=9600 before upload your code, because sim7600 set 115200 by default then save your setting by at&amp;w.

hope this help someone.