After spending a couple of hours on the phone with Sierra Wireless/Semtech Tech Support last week, I ended up being issued and RMA and returning the device for repair/replacement. I am currently waiting to receive an update on that RMA.
For what it’s worth, I did not try the exact steps described in the reply above before returning the device. However, I did make several similar attempts to reset the radio module firmware, mostly following instructions in this forum post and other posts in that same thread. I did not have another SIM card to hand so was not able to try swapping out the SIM. However, my attempts included removing and reinstalling the Verizon firmware we were using, swapping to radio module firmware for a different provider (e.g. ATT, Generic, Sprint) and then back to Verizon. Neither of these methods revived the device in this case. On some occasions, it was impossible to follow these steps because when the device booted up after a reboot, the list of radio module firmware on the Admin → Radio Module Firmware page was blank with no firmware modules listed. Very strange behaviour and did not seem to be consistent or reproducible.