Use N66U as intranet router only -- do not want WAN

[squirrelflower]

Trying to set up a small medical lab with an asus router as an intranet only - no internet. The lab is in a resource poor setting overseas, so internet is not an option. We want the client machines to have DHCP, so we set it up as a router. Unfortunately, whenever the clients access the intranet they get an ugly error message saying that the ethernet cable is unplugged. Disabled WAN, but the error message remains. Thoughts on how to fix this so that the router doesnt care about external internet connection?

Any help is appreciated!
Jan