The AP has DD-WRT on it, as I understand correctly?
It seems like you still do some sort of routing because you talk about WAN and LAN interface, just go into the DD-WRT configuration and make the WAN port part of the switch. Then it will just be an AP, no WAN interface.
Proceed to assign the IP address that will be only for the AP's managment purposes, for clients it will be transparent as they all sit on the same subnet.
If you can't do that, plug the network cable coming from Zentyal in one of the lan ports. Then it is bridged.
I have a DIR-300 flashed with DD-WRT and it is only AP. Clients get their IP from Zentyal and Zentyal is the gateway for the wireless clients.
The DHCP server of DD-WRT will still work (it can hand out IP's on that same LAN segment) but I prefer Zentyal handing out IP's.
Cheers.