Reporting back for future generations, I think I've found the issue. I was retracing my steps and found this: When installing postmarketOS there is a device specific instructionto run "fastboot oem uart enable" command. So I tried to run "fastboot oem uart disable" and then flash UT. So far working like a charm. Did a switch off, did a restart and all works as I'd expect it - also with Libertine set up.

@treebeard : Thanks for sharing the reason! Hopefully it will help others to prevent this experience.

What does exacly exec do? I ask, because sh doesn't work for my script like . does. I'm also interested as I want to run VPN on boot and keep it always on. Here are my scrips and other information. Also, what are the nmcli commands that enable cellular data? I tried to find it and I'm run out of ideas.