TNObject hasn't been instantiated at this point. You are the one that's creating the object in your prefab.Instantiate() call. TNObject doesn't get added/set until after the RCC exits. This is why you're returning the game object reference after all.
If you need to do something that checks tno.isMine, do it in the script's Start() function attached to that instantiated object, or pass the instantiating player's ID alongside other arguments to your RCC call.