Photon Interest Groups for Culling

I must be missing something. The concept of this seems simple enough but it's just not working.

I am doing this globally at the beginning of the Photon session ( and during runtime as needed):

//This should mean we send to peers in our own area and close areas
PhotonNetwork.SetSendingEnabled(disableSend.ToArray(), enableSend.ToArray());

//This is for receiving. EnableInterest just has one area - mine. DisableInterest has all other areas
PhotonNetwork.SetInterestGroups(disableInterest.ToArray(), enableInterest);

Firstly I get an error: Operation ChangeGroups (248) not allowed on current server (MasterServer)
Then the error goes away, even when the code runs again. But the culling has no effect.

If anyone has any ideas....

You apparently call these methods while the client is still doing matchmaking. You first have to get into a room, to use this.
If you didn't, first do the PUN Basics Tutorial.

Thank you Tobiass. I have an existing game, so know how to get into a room. However examples and documentation point to being able to change interest groups on the fly or else not much point using for culling.