Hi
Maybe you need to try this:
https://wiki.nethserver.org/doku.php?id=virtual_network_interface&s[]=dummy
I had only one NIC available with only one IP from the hoster. With the instruction it worked, even for VPNs. (OpenVPN and IPsec).
I suppose / assume you started with a standard / minimal Centos / and installed NethServer over that? AFAIK, only DigitalOcean offers NethServer directly.
What are you using for LAN IP, if it’s in the cloud?
If possible, send a screenshot how the NICs are configured (RED & GREEN).
Can you access it via the dashboard or the newer cockpit?
Andy