[drblah]

As someone who as also spent way more hours on LTE/5G modems than I want to, this post resonates with me. I also learned a few things, namely about the ECM. I will have to look into that. Thanks!

As for the firmware. It seems to just be how these companies do things, unfortunately. At work, we use modems from both Quectel and Simcom and the experience described in OP is very similar. For Simcom we have gotten access to a "secret-but-public" Onedrive share with documentation, drivers and firmware where they put up stuff sporadically. Sometimes the firmware updates even works!

I have a theory that Qualcomm who makes the chipsets used by Quectel and Simcom (at least for the 5G modems) imposes some kind of heavy NDA which prevents free distribution of firmware and documentation. Everything seems very "hush, hush" at least.

While I can't help answer your remaining questions, I can point to wiki I have found really helpful: http://trac.gateworks.com/wiki/wireless/modem