Why my client does not receive data from server that is sent from interrupt function? - arduino-ide

Question

I have to send data to the client in the interrupt function. But apparently, the client cannot receive the data. The server did get the data sent from the client but the client cannot reply back.
When I try the communication in void loop it is a success. But once I put it in the interrupt, I cannot get the data back. The data that the server has to send is simply a number which will then trigger the led at the client side. It is really helpful if someone can help me figure it out.
This is part of the server code.
void ICACHE_FLASH_ATTR detectsMovement() {
LightValue = LightSensor.GetLightIntensity();
Serial.print("Light: ");
Serial.println(LightValue);
if (LightValue < threshold )
{
Serial.println("MOTION DETECTED!!!");
analogWrite(led, 1023);
startTimer = true;
lastTrigger = millis();
WiFiClient client = server.available();
if (client) {
if (client.connected())
{
digitalWrite(ledPin, LOW); // to show the communication only
(inverted logic)
Serial.println("");
String request = client.readStringUntil('\r'); // receives the
message from the client
Serial.print("From client: "); Serial.println(request);
client.flush();
client.println("1\r"); // sends the answer to the client
digitalWrite(ledPin, HIGH);
}
client.stop(); // terminates the connection with the client
}
}
}
void setup() {
Serial.begin(115200); // only for debug
WiFi.config(ip, gateway, subnet); // forces to use the fix IP
WiFi.begin(ssid, pass); // connects to the WiFi router
while (WiFi.status() != WL_CONNECTED)
{
Serial.print(".");
delay(500);
}
pinMode(motionSensor, INPUT_PULLUP);
// Set motionSensor pin as interrupt, assign interrupt function and set RISING mode
attachInterrupt(digitalPinToInterrupt(motionSensor), detectsMovement, RISING);
server.begin(); // starts the server
pinMode(light, INPUT);
LightSensor.begin();
Serial.println("Running...");
pinMode(ledPin, OUTPUT);
pinMode(led, OUTPUT);
}
And the client code
void loop () {
client.connect(server, 80); // Connection to the server
digitalWrite(ledPin, LOW); // to show the communication only (inverted logic)
Serial.println(".");
client.println("Hello server! Are you sleeping?\r"); // sends the message to the server
String answer = client.readStringUntil('\r'); // receives the answer from the server
Serial.println("from server: " + answer);
if (answer == "1")
{
Serial.println("Success!");
analogWrite(D0, 512);
}
client.flush();4
digitalWrite(ledPin, HIGH);
delay(2000); // client will trigger the communication after two seconds
}
The client supposed to get the value '1'. I don't know what to do.