This is slightly offtopic, but I had the problem related to the steps advocated here, and couldn't easily undo everything since you cant undefine mounted tags and /etc/netplan/01-netcfg.yaml did not exists, plus I can't copy/paste via VNC.
If you don't want to reinstall everything, and don't want the useless tag dangling in there/potentially blocking your network, do the following:
Shutdown and rename your original machine
Create a new machine with the name of the original machine (This will use the right Primary vDisk if you set it to auto)
Check if your new Primary vDisk-Location is equal to the original one.
Copy the Network MAC from the original machine to the new machine (I'm not sure if this is strictly required, but this way I didn't need to reconfigure DHCP)
Run the new machine, delete the original machine (DO NOT DELETE THE DISK)
