Let the LMCE core be the gateway. Set the DMZ to the IP of your LMCE server.
Then, go to wizard > remote access, and select the checkbox for [ ] Web site access on port 80.
You can then access the web orbiter from the outside. But you must set up the network as we describe. Either connect directly to the modem, or if you must have a router in the way, let LMCE be the DMZ and connect all of the other computers to the second network card on your core, via a switch.