You are Welcome, that’s this forum is for.
Thinking about your setup I can only recommend to reconsider it once more and invest in at least one extra NIC to separate wan (red) from lan (green) completely. Your setup looks very vulnerable to me, especially your fallback modem worries me.
Also the location of your firewall and Network Allocation Table is unclear, I assume it is in your internet router.
It is good to understand nethserver will act as an firewall/gateway if you set up a red and green interface. This means (as @robb said) red and green must have different subnets.
http://docs.nethserver.org/en/v7/firewall.html
By the way is the subnetmask of your green interface (eth0) of 255.255.252.0 (192.168.xx.0/22) intentional? (it will boardcast on a wider subnet as xx, maybe here your ping is lost.)
Note :
You are planning to setup an nethserver-dc, in your network configuration it means you have to setup dns records in your router. For a easy setup of nethserver-dc it is highly recommended to setup your nethserver as DHCP/DNS server aswell
good luck 