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I'm making a Wireless Notice Board for my project purpose. Simply the expected working is... "Sending a message from our mobile handset to sim which inserted to SIM900A GSM Module Arduino read the message and display through P10 LED display."

Recieving SMS system and P10 LED display are working separately. These programs are combined. SMS are received (we can see in serialmonitor). But it cannot display. Displaying what is written in the program only

#include <SoftwareSerial.h>
#include <SPI.h>
#include <DMD.h>
#include <TimerOne.h>
#include "SystemFont5x7.h"
#include "Arial_black_16.h"

#define ROW 1
#define COLUMN 1
#define FONT Arial_Black_16

DMD led_module(ROW, COLUMN);

String st; 
const char *msg;
unsigned int LengMsg;
SoftwareSerial mySerial(3, 2);

void scan_module(){
  led_module.scanDisplayBySPI();
}

void setup() {
  Timer1.initialize(2000);
  Timer1.attachInterrupt(scan_module);
  led_module.clearScreen( true );
  
  Serial.begin(4800);
  mySerial.begin(4800);
  Serial.println("Starting..."); 
  delay(1000);
  mySerial.println("AT");
  pollSms();
  mySerial.println("AT+CMGF=1"); 
  pollSms();
  mySerial.println("AT+CNMI=1,2,0,0,0");
  pollSms();
}

void loop(){
  pollSms();
  delay(500);

  //Start print in led matrix 
  led_module.selectFont(FONT);
  led_module.drawMarquee(msg,LengMsg, (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 pollSms(){
  delay(500);
  while (Serial.available()) {
    mySerial.write(Serial.read());
    String st = String(Serial.read());
    Serial.println(st);
  }
  while(mySerial.available()) {
    Serial.write(mySerial.read());
    st = String(Serial.readString());
    Serial.println(st);
    LengMsg = st.length();
    st.toCharArray(msg,LengMsg);    
  }
  delay(500);
}