What is the role of my machine's Application pool in Windows service development when consuming third party rest services

Question

I am developing Windows Service in VS2017. While debugging service, first 2 3 calls are working after that getting below errors

System.Net.WebException: 'The remote server returned an error: (503) Server Unavailable.'

The remote server returned an error: (429)

I have tried many things increasing timeout, adding keepAlive etc.. nothing workout. In few articles I found Application pool in IIS that could help me.

But I am not sure if any role of my machine's application pool in windows service when service consuming third party rest API and getting data.

This is my code

private void timer1_Tick(object sender, ElapsedEventArgs e)
 {
    string jsonString = "";
    string jsonstring2 = "";
    string prodfetchurl = HOST;
    string prodfetchurl1 = "testURL";

    var req =
        WebRequest.Create(prodfetchurl) as HttpWebRequest;

    req.Method = "GET";
    req.KeepAlive = true;
    InitializeRequest(req);
    //req.Proxy = null;
    req.Accept = MIME_TYPE;
    System.Threading.Thread.Sleep(200000);
    var response = (HttpWebResponse)req.GetResponse();
    WriteToFile("First service called...");
    if (response.StatusCode == HttpStatusCode.OK)
    {
        Stream responseStream = response.GetResponseStream();
        StreamReader responseReader = new StreamReader(responseStream);
        jsonString = responseReader.ReadToEnd();
    }

    var deserialsseobj = JsonConvert.DeserializeObject<ProductList>(jsonString).Products.Where(i => i.Failed > 0).ToList();
    foreach (var a in deserialsseobj)
    {
        var pid = a.ID;
        string url = FailedDevicesUrl + pid.Value + "/failed";
        var req2 = WebRequest.Create(url) as HttpWebRequest;
        req2.Method = "GET";
        req2.KeepAlive = true;
        InitializeRequest(req2);
        //req2.Proxy = null;
        req2.Timeout = 300000;
        req2.Accept = MIME_TYPE;     
        System.Threading.Thread.Sleep(200000);

        var response1 = (HttpWebResponse)req2.GetResponse();
        Stream responsestream2 = response1.GetResponseStream();
        WriteToFile("Second service called...");
        if (response1.StatusCode == HttpStatusCode.OK)
        {
            StreamReader responsereader1 = new StreamReader(responsestream2);
            jsonstring2 = responsereader1.ReadToEnd();
        }
        var output = JsonConvert.DeserializeObject<List<FailedDeviceList>>(jsonstring2);  // Will get List of the Failed devices
        AutoReprocess(pid.Value, output);
        List<int> deviceids = new List<int>();
        Reprocessdata reproc = new Reprocessdata();
        Reprocessdata.DeviceId rprod = new Reprocessdata.DeviceId();

        reproc.ForceFlag = true;
        reproc.ProductID = pid.Value;
        foreach (var dd in output)
        {
            rprod.ID = dd.DeviceId;
            reproc.DeviceIds.Add(rprod);
        }
        // Reprocess the Product in Devices
        var req3 = WebRequest.Create(ReprocessUrl) as HttpWebRequest;
        req3.Method = "POST";
        InitializeRequest(req3);
        req3.Accept = MIME_TYPE;
        req3.ContentType = "application/json";
        using (StreamWriter writer = new StreamWriter(req3.GetRequestStream()))
        {
            string json = new JavaScriptSerializer().Serialize(reproc);

            writer.Write(json);
            writer.Close();
        }
        var response5 = (HttpWebResponse)req3.GetResponse();
        WriteToFile("Third service called...");
        if (response5.StatusCode == HttpStatusCode.OK)
        {
            string result;
            using (StreamReader rdr = new StreamReader(response5.GetResponseStream()))
            {
                result = rdr.ReadToEnd();
            }
        }
    }
    response.Close();
 }

Methods used in above code

public void AutoReprocess(int pid, List<FailedDeviceList> output)
{
    List<int> deviceids = new List<int>();
    Reprocessdata reproc = new Reprocessdata();
    Reprocessdata.DeviceId rprod = new Reprocessdata.DeviceId();
    reproc.ForceFlag = true;
    reproc.ProductID = pid;
    foreach (var dd in output)
    {
        rprod.ID = dd.DeviceId;
        reproc.DeviceIds.Add(rprod);
    }
    var req3 = WebRequest.Create(ReprocessUrl) as HttpWebRequest;
    req3.Method = "POST";
    req3.KeepAlive = true;
    InitializeRequest(req3);
    req3.Accept = MIME_TYPE;
    req3.Timeout = 300000;
    req3.ContentType = "application/json";
    using (StreamWriter writer = new StreamWriter(req3.GetRequestStream()))
    {
        string json = new JavaScriptSerializer().Serialize(reproc);

        writer.Write(json);
        writer.Close();
    }
    System.Threading.Thread.Sleep(100000);
    var response5 = (HttpWebResponse)req3.GetResponse();
    WriteToFile("Third service called...");
    if (response5.StatusCode == HttpStatusCode.OK)
    {
        string result;
        using (StreamReader rdr = new StreamReader(response5.GetResponseStream()))
        {
            result = rdr.ReadToEnd();
        }
    }
}

public void InitializeRequest(HttpWebRequest request)
{
    request.Headers.Add("aw-tenant-code", API_TENANT_CODE);
    request.Credentials = new NetworkCredential(USER_NAME, PASSWORD);
    request.KeepAlive = true;
    //request.AddRange(1024);
    //request.Proxy = null; 
}

Please help me regarding this.