[How to set IP address without WiFi or DHCP]

Changeing the interface file didn't work for me as well.

My simple solution was to edit the /etc/rc.local and add these lines.

To overwrite the current IP address:

sleep 100

ifconfig eth0 192.168.1.123 netmask 255.255.255.0

route add default gw 192.168.1.1 eth0

The 100 sec sleep is because the search for DHCP IP needs to be finish before the IP "overwrite" can be done.

the RC.LOCAL will be executed every restart of the Ultimaker3