Is there a way to create a shared variable between all the clients over network?

I am developing a racing game project in which i want to do something as simple as when any one of the client hit the finishing point display a message on all other clients showing the name of that specific player say 'John; This is the message that I want to display on all the clients connected when John hits finishing point. 'John has won' After that there is a driving script attached to all the clients which i want to disable. I've been trying to do this for 2 days now and can't seem to handle this scenario. I came to know that client can only update it's instance on other clients and cannot change their's. This is what I've done so far

     [SyncVar] string winner = "";
 
     private void OnCollisionEnter(Collision collision)
     {
         if (collision.gameObject.tag.ToLower().Equals("finish"))
         {
             winner = pname;
             CmdServerNotify(winner);
         }
     }
     [Command] public void CmdServerNotify(string name)
     {
         winner = name;
     }
     [ClientRpc] public void RpcUpdateWinner(string name)
     {
         winner = name;
     }
     void Update()
     {
         if (winner == "")
         {
             GetComponentInChildren<TextMesh>().text = pname;
             if (isLocalPlayer)
             {
                 Camera.main.transform.position = cameraplace.transform.position;
                 Camera.main.transform.rotation = cameraplace.transform.rotation;
             }
         }
         else
         {
             RpcUpdateWinner(winner);
             GetComponent<drive>().enabled = false;
             GetComponentInChildren<TextMesh>().text = winner + " has won.";
         }
     }

This just works for own instance of 'John' on other clients but not their's What is the most appropriate way to do this? Any help will be appreciated