From what you wrote it’s a pretty simple network design.
Put the Nethserver behind your providers router/modem. I’d recommend to set one of your IPs as static IP on red interface. If you have a router in front of your Nethserver you may use port forwarding to forward specific ports (web, mail, …) to the Nethserver or some routers provide a “DMZ host” setting, forwarding all ports to Nethserver.
To be reachable from outside you need a domain (dyndns) domain pointing to your Nethservers public IP.
Naming hints for AD and DNS:
https://docs.nethserver.org/en/v7/accounts.html#dns-and-ad-domain
I recommend letsencrypt to have a valid certificate to avoid problems connecting to your server.
The network clients should use the Nethserver as DHCP and DNS server.
router/modem <-> Nethserver (red) - Nethserver (green) <-> clients
Feel free to ask if you need more details.