why make it difficult?

void loop() {
  //Read SIM800 output (if available) and print it in Arduino IDE Serial Monitor
  if (serialSIM800.available()) {
    Serial.write(serialSIM800.read());
  }

  //Read Arduino IDE Serial Monitor inputs (if available) and send them to SIM800
  if (Serial.available()) {
    s=Serial.read();
    if (s == ';')
      serialSIM800.write(26);
    else
      serialSIM800.write(s);
  }