The DHCP server is enabled by default and dynamically assigns TCP/IP parameters to client devices from the IP Address Pool. DO NOT disable DHCP server unless you have another DHCP server, or you want to manually assign the TCP/IP parameters to every client device on the network.
If you want to set up IP address reservation and assign a fixed IP address to a specific client, please go to "Security" ->"IP/MAC Binding" to set.