This problem is reported in Ubuntu 18.04 generally. The fix hack I am using is to add "@reboot /sbin/dhclient" to the crontab. (sudo crontab -e). Network manager is getting like alsa (and systemd) -- more trouble than it is worth, in all use cases except Lennart Poettering's laptop.
Thanks, Igor. Instructions are a little bit hidden, two-thirds of the way down the "Getting Started" page: Time to try again.