While manually executing script to reflash Sierra EM7455 (since automated steps didn't work), I accidentally skipped a command - one to change USB VID. For those not familiar with USB, there are two numbers - VID and PID - combination of which allows for system to recognize your device. If you change one and not the other, your system will not recognize device anymore.
In my case, I have created a device
413c:9070. It was a monster with VID belonging to Dell and PID of a generic Sierra device. Combination that's definitely not recognized. And I couldn't correct it because that would require serial connection to the device. Serial connection I didn't have due to the device not being recognized. In Windows you're officially screwed. But Ubuntu comes to rescue.
In Linux it's trivial to convince system your "hybrid" device is a valid one. Just load
option module and tell your serial driver there's a new kid in town. In my case, this looked something like this:
sudo modprobe option
echo 413c 9071 | sudo tee /sys/bus/usb-serial/drivers/option1/new_id
With this I got my device's connection back at
ttyUSB2 and could use
minicom to manually change VID:
sudo minicom -b 115200 -D /dev/ttyUSB2