When installing Ubuntu with root on ZFS (following this great guide), I soon discovered that something was amiss with the networking. Wi-fi worked fine, but my mobile broadband failed to connect, and NetworkManager didn’t respond when I plugged in an ethernet cable.
After literally HOURS of troubleshooting (beginning in the wrong end with the mobile broadband), I stumbled across this bug report. Sounds familiar, I installed the ZFS root with a chroot (debootstrap). Thankfully, the comments have the solution.
Create an empty file, and restart stuff.
$ sudo touch /etc/NetworkManager/conf.d/10-globally-managed-devices.conf
$ sudo systemctl restart NetworkManager ModemManagerVoilà! Problem solved.