Do you have Double NAT on your network? Then you have 2 firewalls/gateways to configure.
Internet - ISP router - NethServer - Local Lan
External IP addres on RED interface of ISP router
internal IP address on Green interface of ISP router 192.168.178.0/24 subnet
External IP address on RED interface of NS 192.168.178.23
internal IP address on GREEN interface NethServer (subnet for local LAN)
If possible you could ask your ISP to put the router in bridge mode so your NS gets an external IP address
otherwise you have to do the correct portforwarding on your ISP router and also open the ports on the ISP router to make the connection. Theoreticly double NAT shouldn't be a showstopper.
/edit: I should read the whole thread before posting... lol