Scenes changing problem!

I followed the tutorial on Youtube to setup a menu and the disconnect button in Game scene, and found out that the code not work properly (due to OnLevelWasLoaded ) . How to correct the code and make the disconnect button in another game scene work?

 public void StartupHost()
 {
     SetPort();
     NetworkManager.singleton.StartHost();
 }

 public void JoinGame()
 {
     SetIPAddress();
     SetPort();
     NetworkManager.singleton.StartClient();
 }

 void SetIPAddress()
 {
     string ipAdress = GameObject.Find("InputFieldIPAddress").transform.Find("Text").GetComponent<Text>().text;
     NetworkManager.singleton.networkAddress = ipAdress;
 }

 void SetPort()
 {
     NetworkManager.singleton.networkPort = 7777;
 }

 void OnLevelWasLoaded(int level)
 {
     if (level == 0)
     {

         SetupMenuButtons();
     }
     else
     {
         SetupOtherSceneButtons();
     }
 }

 void SetupMenuButtons()
 {
     GameObject.Find("ButtonStartHost").GetComponent<Button>().onClick.RemoveAllListeners();
     GameObject.Find("ButtonStartHost").GetComponent<Button>().onClick.AddListener(StartupHost);

     GameObject.Find("ButtonJoinGame").GetComponent<Button>().onClick.RemoveAllListeners();
     GameObject.Find("ButtonJoinGame").GetComponent<Button>().onClick.AddListener(JoinGame);
 }
 void SetupOtherSceneButtons()
 {
     GameObject.Find("ButtonDisconnect").GetComponent<Button>().onClick.RemoveAllListeners();
     GameObject.Find("ButtonDisconnect").GetComponent<Button>().onClick.AddListener(NetworkManager.singleton.StopHost);
 }
 

edit: I 've tried adding public void StopGame() { SceneManager.LoadScene(0); } to disconnectButton in Game scene , but all buttons overlapped as the scene went back to Menu.