I strenuously recommend you obtain the Administrator guide for the specific version of ZoneDirector software your ZD1200 is running.
Here is an excerpt from version 10.3 Administrator Guide:
Ruckus recommends that you only enable the built-in DHCP server if there are no other DHCP servers on the network.
ZoneDirector's internal DHCP server can service only a single subnet (the one it's in) and not other VLANs that may be associated with client WLANs. If you enable the built-in DCHP server, Ruckus also recommends enabling the rogue DHCP server detector. For more information, refer to Rogue DHCP Server Detection on page 272.
Go to System > System Settings.
In the DHCP Server section, select the Enable DHCP Server check box.