Been a long time, I already sold my EM20, only using GL-MiFi with original EP-06 for backups and internet device replaced with a SIMCOM 5G module connected to a Raspberry Pi 4b with a 18650 battery for portability, easier to configure drivers in ARM linux.
Before I used Rooter and they added MBIM support for the EM20 on the GL-MiFi and you can communicate AT commands from the ttyUSB2 port. Never got the EM20 to work in either MBIM and QMI on GL-iNet firmware. Not very knowledgeable with building drivers but you can probably create a same firmware with EM20 drivers.