I removed with the nm-connection-editor what I configured with the name “usb0” and did on cmd line from the terminal app:
purism@pureos:~$ nmcli connection add type ethernet ifname usb0 ipv4.method shared con-name local
purism@pureos:~$ nmcli device
DEVICE TYPE STATE CONNECTION
usb0 ethernet connected local
wlan0 wifi connected tarara
p2p-dev-wlan0 wifi-p2p disconnected --
lo loopback unmanaged --
purism@pureos:~$ ip -o addr show usb0
2: usb0 inet 10.42.0.1/24 brd 10.42.0.255 scope global noprefixroute usb0\ valid_lft forever preferred_lft forever
2: usb0 inet6 fe80::f33f:f4d6:688f:9f8d/64 scope link noprefixroute \ valid_lft forever preferred_lft forever
On the laptop I used the devd to start on plugin of the cable dhclient ue0 and this gets an IP addr from the L5:
# dhclient ue0
DHCPREQUEST on ue0 to 255.255.255.255 port 67
DHCPACK from 10.42.0.1
bound to 10.42.0.28 -- renewal in 1800 seconds.
and SSH to purism@10.42.0.1 is fine too.
Thanks for the hints, @dos