connecting modbus tcp with serial communication

#ifdef ESP8266
 #include <ESP8266WiFi.h>
#else //ESP32
 #include <WiFi.h>
#endif
#include <ModbusIP_ESP8266.h>
ModbusIP mb;
int a1 = 0;
int b1 = 0;
void setup() {
  Serial.begin(9600);
  WiFi.begin("SSID", "PASS");//replace with WiFi name & password
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.println("WiFi connected");
  Serial.println("IP address: ");
  Serial.println(WiFi.localIP());
  mb.server();
  mb.addHreg(0);  //input - i
  mb.addHreg(1);  //color - a
  mb.addHreg(2);  //enum/state - b
}
void loop() {
  mb.task();
  int a = mb.Hreg(1);
  if (a1 != a) {
    Serial.println(a);
    a1 = a;
  }
  int b = mb.Hreg(2);
  if (b != b1) {
    Serial.println(b + 10);
    if (b == 0) a1 = 0;
    b1 = b;
  }
}
void serialEvent() {
  while (Serial.available()) {
    int i = Serial.parseInt();
    if (Serial.read() == char(13)) {
      mb.Hreg(0, i);
    }
  }
}