Networkmanager shows WiFi but not Ethernet

My Ubuntu 20.04 laptop did not show an entry for enp0s25, only for wireless connections. This way, I could not connect to a VPN unless I was using the wireless connection that NetworkManager actually saw.

This question has an answer that fixes the issue. Short version:

/etc/network/interfaces:

auto lo
iface lo inet loopback

Add this to /etc/NetworkManager/NetworkManager.conf:

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

Restart NetworkManager, and the ethernet interface appears again!