If you’ve set a static IP with netplan on Ubuntu (or another Linux) and then install a GUI such as ubuntu-desktop or network-manager, the interface can end up outside NetworkManager’s control, so the network won’t connect or you can’t configure it from the GUI. When that happens, create a YAML file in /etc/netplan/ and add a setting so that netplan uses NetworkManager. Something like the following should get it working.

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network:
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  version: 2
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  renderer: NetworkManager