It seems that some LTE dongles that have the correct chipset may not have debug mode enabled, preventing you from entering bootloader mode. Nazmul Alam Nayan provided a tip on how get around this.
Open the casing, there should be a switch. Plug in your device, wait a few moments till it turns on.
Run this command in the terminal
adb wait-for-any-recovery && adb reboot bootloader
Press and hold the switch, the lights will change color/flicker a few times. Let go of the switch and after a few moments your device should go to bootloader mode. From there you can resume the process mentioned in Extrowerks article.