Firebase Connectivity in ESP8266 NodeMCU

I am working on an ESP8266-based IoT project (waste segregation system) and trying to send data to Firebase Realtime Database. I am using the latest Firebase ESP8266 Client library (FirebaseESP8266.h) on Arduino IDE.

I have configured my Firebase database to public/test mode, meaning it does not require authentication, and I want to write simple integer values to it. My ESP8266 is successfully connecting to Wi-Fi, but writing to Firebase fails.

Hardware:

• ESP8266 (NodeMCU or generic ESP-12E)
• Wi-Fi network (SSID provided, password optional)

Firebase setup:

• Realtime Database: public access enabled (test mode)
• No auth key required since public mode )

What I have tried:

#include <ESP8266WiFi.h>
#include <FirebaseESP8266.h>

const char* ssid = "mySSID";
const char* password = "";

#define FIREBASE_HOST "myURL"

FirebaseData fbData;
FirebaseConfig fbConfig;

void setup() {
  Serial.begin(115200);
  delay(2000);

  WiFi.begin(ssid, password);
  Serial.print("Connecting");
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println();
  Serial.println("✅ Wi-Fi Connected");

  fbConfig.host = FIREBASE_HOST;
  fbConfig.api_key = "";  // left blank for public DB

  // For public DB, I pass nullptr as auth
  Firebase.begin(&fbConfig, nullptr);
  Firebase.reconnectWiFi(true);
}

void loop() {
  if (Firebase.setInt(fbData, "/testValue", millis()/1000)) {
    Serial.println("✅ Data written");
  } else {
    Serial.println("❌ Failed to write:");
    Serial.println(fbData.errorReason());
  }
  delay(5000);
}

Observed behavior / error messages:

Serial monitor shows:

✅ Wi-Fi Connected
❌ Failed to write:
token is not ready (revoked or expired)
❌ Failed to write:
token is not ready (revoked or expired