Works but then goes offline

/////////////////////////////////
// Generated with a lot of love//
// with TUNIOT FOR ESP8266     //
// Website: Easycoding.tn      //
/////////////////////////////////
#include <ESP8266WiFi.h>

String  ClientRequest;
IPAddress staticIP483_6(192,168,1,9);
IPAddress gateway483_6(192,168,1,1);
IPAddress subnet483_6(255,255,255,0);

WiFiServer server(80);

void setup()
{
  pinMode(16, OUTPUT);
  ClientRequest = "";

  Serial.begin(115200);
    digitalWrite(16,HIGH);

  WiFi.disconnect();
  delay(3000);
  Serial.println("START");
   WiFi.begin("SSID","PASSWORD");
  while ((!(WiFi.status() == WL_CONNECTED))){
    delay(300);
    Serial.print("..");

  }
  Serial.println("Connected");
  WiFi.config(staticIP483_6, gateway483_6, subnet483_6);
  Serial.println("Your IP is");
  Serial.println((WiFi.localIP()));
  server.begin();

}


void loop()
{

    WiFiClient client = server.available();
    if (!client) { return; }
    while(!client.available()){  delay(1); }
    ClientRequest = (client.readStringUntil('\r'));
    client.flush();
    if (ClientRequest.indexOf("LED=ON") > 0) {

      digitalWrite(16,LOW);
      delay(900000);
      digitalWrite(16,HIGH);


    }
    if (ClientRequest.indexOf("LED=OFF") > 0) {
      digitalWrite(16,HIGH);

    }
    client.println("HTTP/1.1 200 OK");
    client.println("Content-Type: text/html");
    client.println("");
    client.println("<!DOCTYPE HTML>");
    client.println("<html>");
    client.println("<head>");
    client.println("</head>");
    client.println("<body>");
      client.println("<h1 style=""color:#ff0000"">");
      client.println("Sprinkler Timer");
      client.println("</h1>");
      client.println("<a href=");
      client.println("LED=ON");
      //client.println("><button>");
      client.println("><button style=\"height:200px;width:200px\">");
      client.println("Start Timer");
      client.println("</button></a>");

      client.println("<a href=");
      client.println("LED=OFF");
      //client.println("><button>");
      client.println("><button style=\"height:200px;width:200px\">");
      client.println("15 Minutes");
      client.println("</button></a>");

    client.println("</body>");
    client.println("</html>");
    client.stop();
     delay(1);

}