String received through Arduino Serial truncated

Question

I am working on a GSM based electronic notice board using p10 module and SIM800l GSM module.

Due to conflict between the DMD library and the Arduino softwareSerial library, I decide to use two Atmega328p microcontrollers. One microcontroller receives SMS from the GSM module and sends it to the other microcontroller via serial interface.

The issue here is that, when the message is received by the second microcontroller from the first microcontroller, the message is not complete; it is truncated.

Code for receiving SMS and sending the message to the second microcontroller:

#include <SIM800L.h>
SIM800L sim800l(2, 3); //Rx, Tx
int LED_PIN = 8;

//Enter the phone number of the person whom you want to send sms
String reply = "Hey! I have got your message";

void handleSMS(String number, String message) {
  int start_index = message.indexOf('*'); // message starts with '*'
  int end_index = message.indexOf('#'); // message ends with '*'
  //message.trim();
  // if message starts with * and ends with #
  if(start_index == 0 && end_index == (message.length() - 1)) {
    Serial.println(message);
    blinkLed();
    sim800l.sendSMS(number, reply);
  } 
}

void setup() {
  Serial.begin(9600);
  pinMode(LED_PIN, OUTPUT);
  sim800l.begin(9600);
  sim800l.setSMSCallback(handleSMS);
}

void loop() {
  sim800l.listen();
}

void blinkLed(){
   for(int i = 0; i < 3; i++){
      digitalWrite(LED_PIN, HIGH);
      delay(1000);
      digitalWrite(LED_PIN, LOW);
      delay(1000);
 }
}

Code for receiving message from the first microcontroller and displaying it on p10:

#include <SPI.h>
#include  "buzzer.h"
#include <DMD.h>
#include "memory.h"

#include <TimerOne.h>

#include "SystemFont5x7.h"

#include "Arial_black_16.h"


#define ROW 3

#define COLUMN 1

#define FONT Arial_Black_16
char message [250] = "Welcome to electronic projects workshop 1234"; 
char r_msg[250] = "";
bool flag = false;

DMD led_module(ROW, COLUMN);


void scan_module()

{

  led_module.scanDisplayBySPI();

}


void setup()

{
  EEPROM.begin();
  Serial.begin(9600);
  buzzerSetup();
  beeps(3);

  int m_length = EEPROM.read(5);
 
    //    strcpy(message, "");
  String stored_string = read_string(5); 
     
  char stored_char[stored_string.length() + 1];
  stored_string.toCharArray(stored_char, stored_string.length() + 1); // convert message from string to array of char
  strcpy(message, stored_char); // update p10 with stored message

  
  Timer1.initialize(2000);

  Timer1.attachInterrupt(scan_module);

 led_module.clearScreen( true );
writeString(5, "Hello people");
}

void loop()

{
  Serial.println(read_string(5)); 
    receiveMessage();
    
    led_module.selectFont(FONT);

    led_module.drawMarquee(message, strlen(message), (32 * ROW), 0);

    long start = millis();

    long timming = start;

    boolean flag = false;

    while (!flag)

    {

      if ((timming + 20) < millis()) 

      {

        flag = led_module.stepMarquee(-1, 0);

        timming = millis();
      }
    }

  }

void receiveMessage()
{
  /*
    Listens to incoming message through serial interface. 
    It extracts the message if it starts with '*' and ends with '#'
  */
  while(Serial.available())
  {
    String s_msg = Serial.readString();
    // s_msg.trim();
    int start_index = s_msg.indexOf('*') + 1;
    int end_index = s_msg.indexOf('#');
      
    // whether message contains * and # and also # comes after *
    if(start_index > 0 ){
        beeps(1);
        String substr = s_msg.substring(start_index, end_index - 1);
        int m_len = substr.length() + 1;
        char m_msg [m_len];
        substr.toCharArray(m_msg, m_len);
        
        strcpy(message, m_msg);
        // write to eeprom
        writeString(5, substr);
      }
   }
}