Trying to sent notification to multiple users but it working for me only with one user(one token)

Question

This is my code to sent notification of new message in a chat ro all the chats participants:

private inner class SentNotificationToUserAsyncTask(
        val boardName:String,
        val textMessage: String, val token: ArrayList<String>
    )
        : AsyncTask<Any, Void, String>() {

        override fun onPreExecute() {
            super.onPreExecute()
            showProgressDialog()
        }

        override fun doInBackground(vararg params: Any?): String {
            var result: String
            var connection: HttpURLConnection? = null

            try {
                val url = URL(Constans.FCM_BASE_URL)
                connection = url.openConnection() as HttpURLConnection
                connection.doOutput = true
                connection.doInput = true
                connection.requestMethod = "POST"

                /**
                 * Sets the general request property. If a property with the key already
                 * exists, overwrite its value with the new value.
                 */
                connection.setRequestProperty("Content-Type", "application/json")
                connection.setRequestProperty("charset", "utf-8")
                connection.setRequestProperty("Accept", "application/json")

                connection.setRequestProperty(
                    Constans.FCM_AUTHORIZATION, "${Constans.FCM_KEY}=${Constans.FCM_SERVER_KEY}",
                )
                connection.useCaches = false


                val wr = DataOutputStream(connection.outputStream)
                val jsonRequest = JSONObject()
                val dataObject = JSONObject()

                dataObject.put(Constans.FCM_KEY_TITLE,"${boardName}: ")
                dataObject.put(Constans.FCM_KEY_MESSAGE,textMessage )

                jsonRequest.put(Constans.FCM_KEY_DATA, dataObject)
                jsonRequest.put(Constans.FCM_KEY_TO, token)

                wr.writeBytes(jsonRequest.toString())
                wr.flush()
                wr.close()

                val httpResult:Int = connection.responseCode
                if (httpResult == HttpURLConnection.HTTP_OK){
                    val inputStream  = connection.inputStream

                    val reader = BufferedReader(
                        InputStreamReader(
                        inputStream)
                    )

                    val sb = StringBuilder()
                    var line: String?
                    try {
                        while (reader.readLine().also{ line=it} !=null ){
                            sb.append(line+"\n")
                        }
                    }catch (e: IOException){
                        e.printStackTrace()
                    }finally {
                        try {
                            inputStream.close()
                        }catch (e: IOException){
                            e.printStackTrace()
                        }
                    }
                    result = sb.toString()
                }else{
                    result = connection.responseMessage
                }
            }catch (e: SocketTimeoutException){
                e.printStackTrace()
                result = "ERROR : "+ e.printStackTrace()
            }finally {
                connection?.disconnect()
            }

            return result
        }

        override fun onPostExecute(result: String?) {
            super.onPostExecute(result)
            hideProgressDialog()
        }
    }

The class that handling the notification:

class MyFirebaseMessagingService: FirebaseMessagingService() {

    override fun onMessageReceived(remoteMessage: RemoteMessage) {
        super.onMessageReceived(remoteMessage)

        Log.d("onMessageReceived","FROM: ${remoteMessage.from}")

        remoteMessage.data.isNotEmpty().let {
            Log.d("onMessageReceived","Message data: ${remoteMessage.data}")

            val title = remoteMessage.data[Constans.FCM_KEY_TITLE]!!
            val message = remoteMessage.data[Constans.FCM_KEY_MESSAGE]!!

            sentNotification(title,message)
        }
        remoteMessage.notification.let {
            Log.d("onMessageReceived","Message notification body: ${it?.body}")
        }
    }

    override fun onNewToken(token: String) {
        super.onNewToken(token)

        Log.e("onNewToken", "Refresh token $token")
        sentResitrationToServer(token)
    }

    private fun sentResitrationToServer(token: String?){

    }

    private fun sentNotification(title: String,messageBody: String){
        val intent = if (FireStore().getCurrentUid().isNotEmpty()) {
            Intent(this, MainActivity::class.java)
        } else {
            Intent(this, signInActivity::class.java)
        }

        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK or
                            Intent.FLAG_ACTIVITY_CLEAR_TASK)
        val pendingIntent = PendingIntent.getActivity(this,
        0,intent,PendingIntent.FLAG_ONE_SHOT)
        val channelId = this.resources.getString(R.string.deafult_notification_channel_id)
        val defaultSoundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_RINGTONE)
        val notificationBuilder = NotificationCompat.Builder(
            this,channelId
        ).setSmallIcon(R.drawable.ic_baseline_android_24)
            .setContentTitle(title)
            .setContentText(messageBody)
            .setAutoCancel(true)
            .setSound(defaultSoundUri)
            .setContentIntent(pendingIntent)
            .setPriority(NotificationCompat.PRIORITY_MAX)

        val notificationManager = getSystemService(
            Context.NOTIFICATION_SERVICE
        ) as NotificationManager

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O){
            val channel = NotificationChannel(channelId,
            "channel MessagePiegon title",
            NotificationManager.IMPORTANCE_DEFAULT)
            notificationManager.createNotificationChannel(channel)
        }
        notificationManager.notify(0,notificationBuilder.build())
    }
}

The notification will work if I would sent one token instead of Array list of tokens, what do I need to change in order to sent notification to few users?

If sombody have an idea it would be very helpful