I'm starting to loose track...
Please start with checking how your "network objects" have been setup in Zentyal. Remove all spaces, underscores and, capitals and - from the names, both object names and members.
This is the first important step if you wan't to have Zentyal assigning a a static IP to client's. I call this Marcus' rule.
Jons iPad
iPod Touch
This is definitely not going to work for static DHCP.
If that doesn't solve your issue:
If I understand the issue is not only with the iPads but all your wireless items? You have two AP's, one is a Dlink DIR-655 and the other is a WRT54G with DD-WRT on it?
The DIR-655 can be used as AP also if you wish. If it is doing router or gateway, it's client's will never get an IP from Zentyal but instead from the DIR-655 if it has it's DHCP server activated.
If you set the wireless clients with a static IP, their gateway will be the DIR-655 and that IP adress will have to fall in the same space as the DIR-655 lan IP.
If other router is a true AP, then you would rather not activate it's DHCP server. Generally, it's not a good idea to have multiple DHCP servers on your network.
Static IP and Zentyal gateway for clients on that AP won't be a problem because no router/firewall is active.
I have two DIR-300 and both have been flashed with DD-WRT. The only issue a ran into were incompatibilities with the WPA2+AES encryption security.
I had to use WPA personal+AES to get everyone on the AP.
At this very moment I have Zentyal handing out IP's for client's connected to the AP. Client isolation has been enabled and all works OK.
Cheers.