To set Static DHCP Address or to perform DHCP Reservation on EnGenius
routers: 

On the Setup Page:

1. Click on System.
2. Go to DHCP.
3. Enable Static DHCP by checking the checkbox.
4. Input the desired DHCP Address long with the MAC address of the client
   device. 
5. Once done, click Add. 
6. Click Apply when Done.