NetworkManager in Ubuntu 19.10 and 20.04 not working

NetworkManager in Ubuntu 19.10 und 20.04 is disabled by default, except for WiFi connections. If you experience any problems with ethernet connections or vlans (including a vlan that might be configured by netplan but doesn't get activated), check out the configuration file 10-globally-managed-devices.conf.

/usr/lib/NetworkManager/conf.d/10-globally-managed-devices.conf:

[keyfile]
unmanaged-devices=*,except:type:wifi,except:type:gsm,except:type:cdma

As you can see, all devices are declared unmanaged by default, except for wifi, gsm and cdma devices. Move the file to /etc/NetworkManager/conf.d and change it to:

[keyfile]
unmanaged-devices=*,except:type:wifi,except:type:gsm,except:type:cdma,except:type:wwan,except:type:ethernet,except:type:vlan

Restart NetworkManager.

sudo systemctl restart network-manager
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