I just installed 18.04.3 LTS in a RPi 3B+.
I want to run it a) headless and b) with wifi as the only network connection.
At startup, though, it does only activate eth0. And then waits for ages until it continues.
I got the configuration of the NetworkManager so far that a "nmcli d connect wlan0" actually connects the RPi to the WLan - but I have not found the trick how to run this automatically (OK, I could put it into the crontab, but that's not the automatism I'm looking for).
/etc/netplan contains only eth0, but it also says its is auto-generated by cloud-init's network configuration, so touching it is not reset-proof. And honestly, I cannot find where or how to make this cloud stuff create a netplan entry for wlan0...