Esp8266 Http communication over Wifi

Goal

I am trying to send temperature data between two Esp8266 modules. The Server reads a temperature value from the analog pin and is supposed to host this data on a website (I don´t know the correct term for that) over a wifi access point that also runs on the Esp. The Client is supposed to receive the temperature value and output it via serial.

What works

I can connect my phone to the access point and access the data on the IP of the access point. I also can connect the client Esp to my home wifi and it outputs code from different websites.

Problem

But when I try to connect it to the Esp wifi, the wifi login/connection works, but the http.GET function outputs -1 which corresponds to the error message "HTTPC_ERROR_CONNECTION_FAILED".

While the client is connected to the wifi of the Esp my phone displays a similar error message.

I also had the problem that the esp could not continually read A0 while using the Wifi, so I had to build in a delay.

Server Code

// Import required libraries
#include "ESPAsyncWebServer.h"
#include "WiFi.h"
#include <math.h>
#include <ESP8266WiFi.h>

//Constants for temperature calculation
double T;
float V_0 = 5.02;
float R_1 = 99700.0;
float a = 283786.2;
float b = 0.06593;
float c = 49886.0;
float set = 30;
int sensorValue;
float voltage;


// Set your access point network credentials
const char* ssid = "ESP32-Access-Point";
const char* password = "123456789";

// Create AsyncWebServer object on port 80
AsyncWebServer server(80);

String readTemp() {
  return String((-1.0/b)*(log(((R_1*voltage)/(a*(V_0-voltage)))-(c/a)))); //Function to calculate temperature from Voltage
}

void setup(){
  Serial.begin(115200);
  Serial.println();
  
  // Setting the ESP as an access point
  Serial.print("Setting AP (Access Point)…");
  // Remove the password parameter, if you want the AP (Access Point) to be open
  WiFi.softAP(ssid, password);

  IPAddress IP = WiFi.softAPIP();
  Serial.print("AP IP address: ");
  Serial.println(IP);

  server.on("/temperature", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", readTemp().c_str());
  });
  
  // Start server
  server.begin();
}
 
void loop()
{
 //reading of A0
 Serial.println(voltage);          
 Serial.println(readTemp().c_str());
 sensorValue = analogRead(0); //?Wifi not working, when reading A0?
 delay(10);
 voltage = sensorValue * (3.3 / 1023.0);
}

Client Code

#include <ESP8266WiFi.h>
#include <ESP8266HTTPClient.h>
#include <WiFiClient.h>

#include <ESP8266WiFiMulti.h>
ESP8266WiFiMulti WiFiMulti;

const char* ssid = "ESP32-Access-Point";
const char* password = "123456789";

void setup () {
 
  Serial.begin(115200);
 
  WiFi.begin(ssid, password);
 
  while (WiFi.status() != WL_CONNECTED) {
 
    delay(1000);
    Serial.println("Connecting..");
 
  }
  Serial.print("Connected to ");
  Serial.println(ssid);
}

void loop() {
 
  if (WiFi.status() == WL_CONNECTED) { //Check WiFi connection status
 
    HTTPClient http;  //Declare an object of class HTTPClient
 
    http.begin("http://192.168.4.1/temperature");  //Specify request destination
    int httpCode = http.GET();                                  //Send the request
 
    if (httpCode > 0) { //Check the returning code
 
      String payload = http.getString();   //Get the request response payload
      Serial.println("/////////////////////////////////////////////////////////////");
      Serial.println(payload);             //Print the response payload
 
    }
    else{Serial.println("Error");}
    http.end();   //Close connection
 
  }
 
  delay(30000);    //Send a request every 30 seconds
}

I would really appreciate it if anybody could help me with this because I have been tearing my hair out for weeks over this. Thanks in advance and sorry for any grammatical or spelling mistakes in advance.