Using RegisterStartPosition () and UnRegisterStartPosition ()

I’m trying to create a “disable this spawn point” script using both RegisterStartPosition() and UnRegisterStartPosition():

using UnityEngine;
using UnityEngine.Networking;
using System.Collections;

public class SpawnDisable : NetworkBehaviour {

	private NetworkManager net;

	[Server]
	void Start ()
	{
		net = GameObject.FindGameObjectWithTag ("NetworkManager").GetComponent<NetworkManager> ();
	}

	[Server]
	void OnTriggerEnter(Collider col)
	{
		net.UnRegisterStartPosition (gameObject.transform);
	}

	[Server]
	void OnTriggerExit (Collider col)
	{
		net.RegisterStartPosition (gameObject.transform);
	}
}

but I’m getting errors from Unity:

Static member 'UnityEngine.Networking.NetworkManager. RegisterStartPosition(UnityEngine.Transform)' cannot be accessed with an instance reference, qualify it with a type name instead

And the exact same error for UnRegisterStartPosition(). This is the code I’m using:

I’ve been Googling around trying to find a solution to the “Static Member” problem but I’ve only found solutions for variables, not a function. I’ve had a look at the source of NetworkStartPosition.cs that uses RegisterStartPosition() but I’m none the wiser.

Does anyone know how to use these functions correctly?

Both of the methods are STATIC that means you access them via the class name like so:
NetworkManager.RegisterStartPosition(transform);