Can't access web interface from clients that are not directly connected

Basic network map:

......................Modem/Router..................
......................../............\.....................
.........Untangle FW...........WiFi Access Point
.............../......................./.................\
...........Switch.................:(..................:(
............/......\.......................................
...........:)......:)....................................

The :)'s and the :('s are both on the same subnet and have no issues connecting to each other. But only the :)'s who are directly connected to the Untangle box can access the web interface. The :('s, who are connected via WiFi directly to the router, can not access the web interface. They can ping the Untangle box though.

The problem goes away when I plug the WAP into the switch, and that's an acceptable solution for me. But I'm curious what would cause this behavior. Does Untangle only allow clients that are directly connected to it access to the web interface? If so, is there a setting to turn this option off?