A machine that is going to act as a DHCP server must have a the FreePBX Firewall correctly configured, with an 'Internet' interface, and a 'Local' interface. Note that multiple interfaces can be enabled for DHCP, but only traffic that originates on a Local network and leaves on a Internet network will be NAT'ed. All other traffic will be passed through untouched.
Make sure that the Local interface does NOT have a default gateway - this machine IS the default gateway for the Local network!
After it is enabled, if you need to change any settings (Note that these are automatically configured and it is extremely unlikely that you will need to change them), this can be done by clicking on the 'Manage' button
Advanced Options (Port forwarding, etc)
The standard DHCP server does not support advanced options (such as Port Forwarding) through the GUI. These can be manually added to /etc/firewall-4.rules file if needed.