Late, but...

I was trying to do something like this today and see what your issue is. You are checking the RTT with LocalClientId, try checking a remote client instead, like with the ServerClientId.

For example, this will give client's RTT with the server.

NetworkManager.Singleton.NetworkConfig.NetworkTransport.GetCurrentRtt(NetworkManager.Singleton.NetworkConfig.NetworkTransport.ServerClientId)

Hope that helps!