You really need to read some basic info on what routing is and what a gateway does. From your story I think you try to configure 2 subnets on the same network interface. This is NOT possible.
If you want NethServer to act as a IDS/Firewall/Gateway you have to create the following situation:
Internet - Modem/Router (192.168.1.1) - (192.168.1.26) RED interface NethServer - GREEN interface NethServer (192.168.0.1) - Switch to Local LAN (subnet 192.168.0.1/24)
As you can see, NethServer MUST be in between your modem/router and the rest of your LAN and your Modem/Router and RED interface MUST be on a different subnet as your GREEN interface.