Hello @whotopia
if you look at the schematic you'll see that the DC input voltage always goes to the step down converter (independent of the S1 input switch position). The input voltage is not 5 V or 12 V - it can be anything from 5 V to 12 V.
(Please note: 5 V is actually a bit low as with 5 V the step down converter only is able to produce about 4.5 V.)
The input voltage switch determines which voltage goes to the green LoRaWAN board. If set to 5 V then the output voltage of the step down converter goes to Vin of the green board. If set to 12 V then whatever voltage is applied externally goes to Vin of the green board.
The green board has a separate voltage regulator which seems to generate 3.3 V for the LoRaWAN chip. (Whether that is the only voltage required I cannot tell as there is no schematic for the green board.)
What you can try is the following: Set the power input switch to 5 V and run the Fire from battery. This should generate 5 V on the MBus which then goes to Vin of the green board.
Please note: While I have other COM.X modules I was able to measure the output voltage of the step down converter when only fed with 5 V. However I do not have the COM.LoRaWAN868 v2.0 to test with.
Thanks
Felix