For some reason, I wrote down the systemsettings5 control panel that doesn’t get you to the network management GUI that you need to control stuff controlled by Network Manager. To configure the actual interfaces, you need nm-connection-editor
You can use nmcli to configure network interfaces controlled by NetworkManager. But, honestly, I don’t see any advantage to learning the cryptic CLI instead of using the cryptic config file stuff I’ve already learned. And yet … I need my server to have /etc/resolv.conf populated when it reboots. So I figured out how to launch the KDE system settings (assuming you’ve got the X display redirected to your host) — systemsettings5
Mine takes a few minutes to render, during which time the window is black and a handful of errors are written out to the console. But it got there eventually, and I was able to edit the network interface.