Esp8266 Http communication over Wifi - http

Question

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.