Hi koroknai,
I’m observing very similar situation with one of mine LtAP mini LTE kit router (one out of ten so far)
Symptoms are the same generally - after reboot or switching modem slot, LTE interface just get lost.
I turned on additional lte-logging and you can see the result on the screenshot.
At 01:14:08 LTE interface was present and ROS had been actively complaining about SIM absence (that was true, but it does not really matter)
Then I tried to switch sim slot (at 01:14:09), LTE interface disappeared (it’s ok) and … nothing then… and it’s NOT OK
As you can see, I had been waiting quite a long time, but almost two hours later… still nothing.
Then I switched sim-slot back - and nothing had happen again
In this state (when lte is absent) usb2 port marked as invalid.
USB power reset does not help.
P.S. I observed the same issue with sim-cards were installed in both slots as well that’s why I think it does not relate to possible sim-card issue.
I tried this slot switching exercise with another LtAP mini that I currently have on board and it goes without any issues.
Here was a suggestion regarding possible power issue when running on microUSB, but I played with powering from microUSB and from standard power supply - all the same.