Esp8266 code (Modified)

esp01_mod.ino

#include <ESP8266WiFi.h>
#include "ESP8266WebServer.h"

char ssid[] = "demowifi";   //  network SSID (name) 
char pass[] = "demowifi";   //  network password

//serial comm variables
uint8_t dataToSend = 0;
char dataFromAtmega[30];

ESP8266WebServer server(80);
String dataFromRequest = "";

void setup() {
  Serial.begin(9600); //serial comm initialized
  WiFi.mode(WIFI_STA); //esp01 initialized in station mode
  
  WiFi.begin(ssid, pass); //connect to internet via WiFi

  while (WiFi.status() != WL_CONNECTED){ 
    delay(500);}     
  
  server.onNotFound(handleHttpRequest);
  server.begin();

}

void handleHttpRequest(){

  dataFromRequest = server.uri(); //the data that comes with the GET request . Format : "/(number)"
  dataFromRequest.remove(0,1); // deleting the '/' from the data for converting string to integer
  dataToSend = dataFromRequest.toInt(); // string to int conversion
  
  //serial communication between Atmega328p and Esp01
  Serial.print(dataToSend);
  while(!Serial.available());

  if(Serial.available())
     Serial.readBytesUntil('#',dataFromAtmega,30); //reads the input until "#"  and stores it in the data array.

  //response and data sent to client 
  server.send(200,"text/plain",dataFromAtmega);
  }

void loop() {
  server.handleClient();
  }