Chip, thanks for the TPL5110 note. I have determined the TPL5110 is not suitable because it isn't really a full power switch. It rather, as you note, can send pulses to control the RST and EN pins. And, I have read enough on these forums to know that manipulating RST - and even EN - can be insufficient to recover from bad states.
Today, I actually got the U6030 working as an external hardware watchdog pursuant to @Rftop 's post here:
You need to put the U6030 on Mode 5 and, sadly, it needs a 12v input (not a problem for my current deployment, but I have some sites which are 6V only).
Does anyone know of a 5V equivalent version of the U6030?
This little device, along with usage of V1.3.1-rc1, appears to be the magic trick allowing me to use Boron LTE in a stable, reliable way.
And although it requires 12V in, I am currently triggering it/petting it with 3.3v logic level digital Boron D8 out - and it is working just great. I have T1 (watchdog interval) set to 300s, and T2 (shutoff/reset duration) set to 30s.
I would love to find the same exact board but able to use 5v or even 3.3v power.