#include <WiFi.h>
#include <ThingSpeak.h>

const char* ssid = "[WIFI 이름]";
const char* password = "[WIFI password]";

...

...
WiFiClient client;
unsigned long ch1_ID = 2126281;
const char* ch1_WriteKey = "AGP30QRISWWCJ0ZO";

void setup() {
  Serial.begin(115200);
  delay(10);
  initWiFi();
  ThingSpeak.begin(client);
}

void loop() {
  int i;
  for (i = 0; i < 4; ++i) {
    ThingSpeak.writeField(ch1_ID, 1, i, ch1_WriteKey);
    delay(2000);
  }
}

void initWiFi() {
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.println(".");
  }
}