Teensy 4.x + SARA-R410 + TinyGSM + SSLClient = failed mTLS connections?

[MaffooClock]

I'm prototyping a project that is designed to communicate with AWS IoT MQTT via cellular IoT. Using the MQTT service through AWS means mTLS is required, and the only library that can provide that for Arduino is OPEnSLab-OSU/SSLClient.

It seems that when combining a u-blox SARA-R410 (or, perhaps more specifically, this SparkFun shield) with a Teensy 4.x, mTLS connections never complete. I opened issue #33 with SSLClient thinking the problem was with the library, but after switching to another modem (same codebase), the problem went away.

I've used the same SARA-R410 with other Arduinos (SAMD21 and SAM3X MCUs) with the same codebase with plenty of success, but for some strange reason, this particular shield seemed to cause a problem with SSLClient on the Teensy 4.x.

The only reason I'm opening this issue is just in case someone else happens to experience the same trouble I did with the same environment. Maybe this is a highly unique situation that only I will ever experience, or perhaps my SparkFun shield is somehow defective (unlikely, seems to work fine otherwise), but I figured I had an obligation to share.

Note: I was not using the official SparkFun LTE Shield library, as I prefer the vshymanskyy/TinyGSM library.