AT+CIPSEND only working for the first time.

Hi,
I'm using Sim7600 and trying to connect to my tcp server, hold connection to send messages back and forth.
This is a part of my code:

while (1) {
      while (myserial.available()) {
        Serial.write(myserial.read());
      }
      
      myserial.println("AT+CIPSEND=0,4");
      delay(3500);
      
      myserial.println("a ok");
      delay(5000);     
      
      while (myserial.available()) {
        Serial.write(myserial.read());//if Serial received data, output it via mySerial.
      }
}


The problem is it only works at the first loop. The message is only send once.  However, when I use serial monitor to send command. I can send AT+CIPSEND as many as I want. 

Hope you guys can help me find out what's wrong.