Using this set of instructions for device tree overlays on a Raspberry Pi, I tried to create an overlay to enable the second serial port on this LTE dongle, making what I thought were the appropriate changes.
When I rebooted, I got lots of errors. One of the final errors told me that the root filesystem couldn't be found, followed by an initrd login prompt.
Not knowing how to undo my changes from here, I decided that the fastest way to fix things was to re-flash the Debian image again.
That didn't start off well, either. ADB couldn't see the device. I rebooted by re-plugging the USB connection, and during the bootup process, I issued adb devices. As soon as a device was detected, I quickly issued adb reboot bootloader and fastboot devices. I was then able to flash the Debian image again.
So, I'm back to square one, but at least I didn't brick the stick. So far, all of my efforts with the software have failed. If anyone more knowledgeable about device tree overlays would like to help, I'd appreciate it.