How to get eth0 and wlan0 back on Ubuntu Mate 18.04?

Sun Jan 31 18:46:41 UTC 2021

>On Sun, 31 Jan 2021 18:23:46 +0100, Bo Berglund wrote:
>>>You aren't managing networks by NetworkManager, Wicd,
>>>systemd-networkd, Netplan, something else?  
>>
>>I don't know...

You should find out, since those have got their own configurations.
The German Ubuntu users Wiki claims that Ubuntu >= 17.10 defaults to
Netplan, https://wiki.ubuntuusers.de/interfaces/ . It might be
irrelevant, when updating an install using something else or perhaps
when updating a tailored minimal install using something else.