By default the network-manager snap has support for Ethernet disabled as Ubuntu Core uses networkd/netplan by default to manage Ethernet connections. As both will naturally conflict, the network-manager snap is currently configured to ignore Ethernet devices and the user has to take care to enable it after installation.
For future versions, we plan to improve this to allow the network-manager snap to perform all necessary steps to enable Ethernet support automatically.
Before doing any of the steps described below, backup the content of /etc/netplan to be able to restore it at a later point.
Also, ensure you do a full backup of the system as your system may end up without any network connection which can lead to problems accessing headless devices when no serial console is available.
Enable Ethernet support by following these steps:
As a first step, install the network-manager snap as described here if it is not already installed.
If you don't want any netplan configuration files being used by NetworkManager, delete all files from /etc/netplan. Any remaining files will be considered by NetworkManager and used to configure Ethernet and wireless network devices. The configuration of those connections can't be changed through the NetworkManager command line utilities or the DBus API.
To tell netplan to render NetworkManager connection configuration files by default, create the file /etc/netplan/00-default-nm-renderer.yaml with the following content:
network: renderer: NetworkManager
Everything is now set up and ready for NetworkManager to take over. After a reboot of the system NetworkManager will manage all Ethernet connections. Reboot the system with
$ sudo reboot
When the system is rebooted NetworkManager should automatically set up attached Ethernet ports or use existing netplan configuration files to setup connections.
Once logged into the system you may check the current connection status by
$ nmcli c show