hi, to make captive portal work, zentyal needs to be dns, dhcp and router/gateway for the network.
to make this work ... this requires it to have an ip/interface in the target network,
to make this work set captive portal on network interface, or a vlan interface... if you want to work on a vlan only you will require lvl3 (smart) switches. otherwise any lvl2 switch will do. if you want it wireless than add a wap (wireless access point) to your network segment if you use consumer based wireless routers, be sure to disable its dhcp and dns functions completly before connecting it to your captiveportal network.