Network Manager refusing to manage wired interfaces [duplicate]
Solution 1:
from NetworkManager 1.2.4 (ubuntu 16.10) all non-wifi/wwan device are treated as unmanaged
[keyfile]
unmanaged-devices=*,except:type:wifi,except:type:wwan
workaround: comment out the content of /usr/lib/NetworkManager/conf.d/10-globally-managed-devices.conf or remove the file
for details see https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/1638842