Sorry @SmurfonToast for the late reply. So according to the blog post on converting the E3372 to stick mode:
This works, however, the use of
/dev/cdc-wdm0device is supposed to achieve better performance than/dev/ttyUSB*.
It'd be a shame that cdc-wdm0 device is removed from the GL-iNet UI. However you can still create the modem interface in LuCI and choose the interface you want.
The E3372 worked fine with the Beryl, but occassionally it'd lose Internet connection so I had to install a Cron job to reestablish it every now and then.
Nevertheless, I purchased a Spitz AX for native support for SIM cards. It's much more stable without all the fiddling, and there's 5G!