I managed to open the SIM slot of the phone and inserted a working SIM from another phone. Since the SIM was not detected at all, I decided to give the nightly CynogenMod 12.1 on the phone. Again via SFTP to SSHDroid in the LAN.
An Optimus G E975 (Intl) was still waiting for wakeup, but official updates stuck at Android 4.4.2.
So I applied CyanogenMod to that phone and since it was not that easy to gather all the necessary information, I write down a short protocol of the steps, that helped in my case. I started with the rather incomplete (with respect to rooting and recovery) howto at wiki.cynogenmod.org.
I recently upgraded my iPhone 5 to an Optimus G, which fits better to the profile of a Linux User and after I was never satisfied with the results of Siri (at least in German), the voice recognition of Google Now is in comparison more than convincing.