Network configuration - howto define interface connected with another LAN

My ISP offers static IPv4 address.

I was thinking to assign NethServer red to this static IPv4 address bypassing the router.
This means the router offers function “bridge” and hereby acts as modem only.

In the end I will have direct internet access with NethServer over static IPv4 + internet access with router using dynamic public IP.

The network behind the router is a private LAN equal to the green network of NethServer.

Question:
How can I connect these two networks?