Network manager has been working reasonably well in Ubuntu and other Debian derivatives for quite some time and then Canonical changes to netplan. This is causing some confusion and is clearly seen by many (myself included) as a step backwards.

The original configuration file residing in /etc/netplan/01-netcfg.yaml contains the following:

# This file describes the network interfaces available on your system
# For more information, see netplan(5).
network:
  version: 2
  renderer: networkd
  ethernets:
    enp0s31f6:
      dhcp4: yes

This leads to the conclusion that reversion to network-manager with its text based interface (nmtui) etc might not be so easy but actually it is. Change the configuration file in netplan to the following:

# This file describes the network interfaces available on your system
# For more information, see netplan(5).
# Set and change netplan renderer to NetworkManager GUI tool 
network:
  version: 2
  renderer: NetworkManager

When done don’t forget to apply the changes to netplan:

$ sudo netplan apply

This takes care of the problem and now you can re-install network-manager with whatever interface you want to use and handle all your network interfaces through that instead.