in transparent/split mode with a dhcp server which is not your UTM the AP10/30/50s don't work for now, but the AP5 does. If you use Standard/Split mode and you have an DHCP Server configured on your UTM for the RED interface, you will have to set the standard gateway for this dhcp server as the red. In "Wireless Protection"->"Global Settings" you have to select the red interface for allowed networks. And use a "bridge to LAN" SSID.