When you installed Cockpit in Debian system and some of your interfaces are showing under the Unmanage interfaces section, the solution is to comment out the intefaces in /etc/network/interfaces. This will make the network-manager manage the interfaces; therefore, Cockpit will be able to manage the interfaces.
root@debian:~# nmcli device status DEVICE TYPE STATE CONNECTION ens18 ethernet unmanaged -- lo loopback unmanaged --
Comment out the physical interfaces in
sed -i 's/allow/#allow/g' /etc/network/interfaces sed -i 's/iface ens18/#iface ens18/g' /etc/network/interfaces
Restart the network-manager
systemctl restart NetworkManager
It is a little something that I encountered. Hope you’ll this informative. Cheers!