Hi Patcunha,
To make DHCP server to work, you must change the default gateway to a custom IP address (your case 192.168.16.1) to point the DHCP clients to the correct path since eBox is not working as gateway.
To recover from that mistake, do the following to flush the iptables rules:
$ iptables -P INPUT ACCEPT
$ iptables -P OUTPUT ACCEPT
$ iptables -F
Then, access to the eBox Web UI and unset the external checkbox and save changes.
Hope this helps.