Ok, quite simple actually : )

open Editor, navigate to /etc/config and edit dhcp file.

At the end of the file, add the following:

config host
option name 'name of your host'
option ip 'XXX.XXX.X.X' // address ip you want to assign this device.
option mac 'XX:XX:XX:XX:XX:XX' // // mac address this device.

you can add as many as you want.

Thanks for your help!