When you want to connect a not-Internet-connected router via ethernet to your Ubuntu laptop, but still prefer your laptop’s Wi-Fi connection for Internet routes, you can delete the default route for the ethernet connection:
$ /sbin/ip route | awk '/default/ { print $3 }'
$ sudo route del default gw 192.168.100.1
^^ where the 192 address is the gw for the ethernet connection
There may be better ways to do this, such as using ifmetric, but the above works for me.