How do I put into a buffer, a part of Serial response (UART)

#include <SoftwareSerial.h>

int baud_rate = 9600;
int pin_gsm = 3;
int pin_gps = 4;
int pin_power = 5;
//int pin_dtr = 6;
boolean debug = true;
boolean raedy_to_go = false;

// Reading String
#define BUFFERSIZE 200
char buffer[BUFFERSIZE];
char inChar;
int index;

void setup()
{
  Serial.begin(baud_rate);
  delay(5000);                         // Wait for 5sec after begin
  
  if(debug)
  {
    Serial.println(F("\n****************************"));
    Serial.println(F("STARTING SYSTEM Read AT stream"));
    Serial.println(F("******************************"));
  }
  pinMode(pin_gsm,OUTPUT);            // Set the pins
  pinMode(pin_gps,OUTPUT);
  pinMode(pin_power,OUTPUT);
 
  digitalWrite(pin_power,HIGH);
  delay(1500);
  digitalWrite(pin_power,LOW);
  // The module is started but I want to be sure it has really started
  // I send an AT command and I should receive an response RDY
  // That function should 
  // 1. Send the AT commans
  // 2. Store the response in a buffer

  if(strstr(read_AT_string("AT",5000),"RDY") != NULL)
  {
    Serial.println(F("\n> Module is started UP"));
    Serial.println(F("------- Show buffer------"));
    Serial.println(buffer);
    Serial.println(F("-------------------------\n"));
    
    // Enabling GSM
    gsm_enable();
    
    Serial.println(F("\n> Powering GPS"));
    Serial.println(F("------- Show buffer------"));
    Serial.println(read_AT_string("AT+CGPSPWR=1",5000));
    Serial.println(F("-------------------------\n"));
    // AT+CGPSPWR, should it return OK
    
    Serial.println(F("\n> reset GPS in autonomy mode"));
    Serial.println(F("------- Show buffer------"));
    Serial.println(read_AT_string("AT+CGPSRST=1",5000));
    Serial.println(F("-------------------------\n"));
    // AT+CGPSRST=1, should it return OK
    
    
  }
  else{
    Serial.println(F("> Module is NOT started UP"));
    Serial.println(F("------- Show buffer------"));
    Serial.println(buffer);
    Serial.println(F("-------------------------\n"));
  }
}

void loop()
{

    
}

char* read_AT_string(char* command, int timeout)
{
  unsigned long previous;
  previous = millis();
  index=0;
  // Clear the buffer
  buffer[0]='\0';
  
  // SEND THE AT COMMAND. I AM ASKING IF I HAVE TO KEEP THE ln
  Serial.println(command);
  do
  {
    if(Serial.available() > 0) // Don't read unless
    {
      if(index < BUFFERSIZE-1)
            {
              //Serial.println("b");
              inChar = Serial.read();  // Read a character
              buffer[index] = inChar;  // Store it
              index++;                 // Increment where to write next
			    // BUFFER CONTAINING THE DESIRED RESPONSE
              buffer[index] = '\0';    // Null terminate the string
            }
    }
  }while(((millis() - previous) < timeout));
  
  return buffer;
}

void gps_enable(void)
{
  if(debug)
  {
    Serial.println(F("\n> Enabling GPS"));
  }
  digitalWrite(pin_gps,LOW);                //Enable GPS mode
  digitalWrite(pin_gsm,HIGH);                //Disable GSM mode
  delay(2000);
}

void gsm_enable(void)
{
  if(debug)
  {
    Serial.println(F("\n> Enabling GSM"));
  }
  digitalWrite(pin_gsm,LOW);                //Enable GSM mode
  digitalWrite(pin_gps,HIGH);               //Disable GPS mode
  delay(2000);
}

buffer[0]='\0';