Photon server keeps kicking me off from room

Hi guys, I'm quite new to Unity and Photon, my problem is whenever I joined a room, I get kicked out from the room after one or two minutes. I saw some threads saying this behaviour is due to too many messages are transferred in a short period of time. I tried removing all the photonViews and networking related components, the result is still the same, I still get kicked out from the room by server. I have no idea where is the error come from, even I open the Windows task manager, it shows no network transmission is occurring. Here is my code:

using UnityEngine;
using Photon.Pun;
using Photon.Realtime;
using System.Collections;
using System.Collections.Generic;
using UnityEngine.SceneManagement;
using Hashtable = ExitGames.Client.Photon.Hashtable;
using ExitGames.Client.Photon;

public class NetworkManager : MonoBehaviourPunCallbacks
{
    public static NetworkManager Instance { get; private set; }

    void Awake()
    {
        if(Instance == null)
        {
            Instance = this;
            DontDestroyOnLoad(gameObject);
        }
        else
        {
            Destroy(gameObject);
        }
    }

    void Start()
    {
        PhotonNetwork.ConnectUsingSettings();
    }

    public void JoinLobby()
    {
        //===== check if the client is connected to master server before joining lobby
        if(PhotonNetwork.NetworkClientState == ClientState.ConnectedToMasterServer)
        {
            PhotonNetwork.JoinLobby();
        }
    }

    public void JoinOrCreateRoom(string roomName, string roomType)
    {
        PhotonNetwork.SetPlayerCustomProperties(UserInfo.localUserInfo.GetCustomProperties());
        
        Hashtable customPropertise = new Hashtable { { "RT", roomType } };
        RoomOptions roomOptions = new RoomOptions()
        {
            IsVisible = true,
            IsOpen = true,
            CustomRoomProperties = customPropertise,
            CustomRoomPropertiesForLobby = new string[] { "RT" },
            MaxPlayers = 0
        };

        PhotonNetwork.JoinOrCreateRoom(roomName, roomOptions, TypedLobby.Default);
   }

    public void JoinRoom(string roomName)
    {
        PhotonNetwork.SetPlayerCustomProperties(UserInfo.localUserInfo.GetCustomProperties());

        PhotonNetwork.JoinRoom(roomName);
    }

    public void LeaveRoom()
    {
        PhotonNetwork.LeaveRoom();
    }


    #region Pun Callbacks
    public override void OnConnectedToMaster()
    {
        base.OnConnectedToMaster();
        Debug.Log("connected to master");

        this.JoinLobby();
    }

    public override void OnJoinedLobby()
    {
        base.OnJoinedLobby();
        Debug.Log("joined lobby");
    }

    public override void OnLeftLobby()
    {
        base.OnLeftLobby();
        Debug.Log("left lobby");
    }

    public override void OnRoomListUpdate(List updatedRoomList)
    {
        base.OnRoomListUpdate(updatedRoomList);
    }

    public override void OnCreateRoomFailed(short returnCode, string message)
    {
        base.OnCreateRoomFailed(returnCode, message);
        Debug.Log("fail to create room");
    }

    public override void OnJoinedRoom()
    {
        base.OnJoinedRoom();
        Debug.Log("Joined room >>>>> " + PhotonNetwork.CurrentRoom.Name);

        //load in room scene
        PhotonNetwork.LoadLevel(PhotonNetwork.CurrentRoom.CustomProperties["RT"].ToString() + " Room"); //RT => Room Type
    }

    public override void OnLeftRoom()
    {
        base.OnLeftRoom();
        Debug.Log("OnLeftRoom");

        //load main screen
        PhotonNetwork.LoadLevel("MainScene");
    }

    public override void OnPlayerPropertiesUpdate(Player target, Hashtable changedProps)
    {
        base.OnPlayerPropertiesUpdate(target, changedProps);

        if (target.IsLocal)
        {
            UserInfo.localUserInfo.UpdateUserInfo(changedProps);
        }
        else 
        {
            for(int i = 0; i < UsersInRoom.Count; i++)
            {
                if (target.NickName == UsersInRoom[i].Username)
                {
                    
                    UsersInRoom[i].UpdateUserInfo(changedProps);
                }
            }
        }
    }
    #endregion
}

Can someone help? I don't mind to share my project if needed. Btw I'm using PUN2 version 2.12, Unity 2018.3.8f1