Apple TV on local LAN only

I"m trying to set up a AppleTV + TV as a display solution for a conference room. The AppleTV cannot be connected to the Internet at this location, but all I want to do is use Airplay to display presentations from an iPad or Mac laptop.

What I've done is set up an old Linksys WiFi router which both the Apple TV and iPad connect to. This part works, both are connected to the router, wirelessly and the iPad can connect to the Linksys admin console via safari.

Still no airplay.

Is there something I need to enable/disable on the Linksys?