Comment 2 for bug 1986444

I followed some suggestions: https://askubuntu.com/questions/906636/ethernet-adapter-was-disable-on-ubuntu-17-04/909185#909185 to fix the problem. Specifically, I edited the file:
/usr/lib/NetworkManager/conf.d/10-globally-managed-devices.conf
and changed the line unmanaged-devices=*,except:type:wifi,except:type:gsm,except:type:cdma
to unmanaged-devices=*,except:type:ethernet,except:type:wifi,except:type:gsm,except:type:cdma

I also followed the suggestion: https://askubuntu.com/questions/1039233/no-wired-connection-wired-unmanaged-ubuntu-18-04/1043244#1043244
and edited the file: /etc/NetworkManager/NetworkManager.conf and changed the line
managed=false to managed=true

After sudo systemctl restart NetworkManager and a reboot the problem seems to be fixed.
Incidentally, there were quite a few problems with the upgrade, and I had to reinstall the
Ubuntu Desktop to fix some of these. I do not know if the NetworkManager files were changed or deleted when I did this.