@ywp hey thanks! I actually found out from this blog that if you force the E3372 to the "modem only" mode (AT^SETPORT="FF;10,12,16"), it will use /dev/cdc-wdm0 instead of /dev/ttyUSB*, which not only increases the performance but also lets the MT3000 recognize the modem properly:
For SMS, I installed luci-app-sms-tool which lets me read/send messages from luci itself.
Now my MT3000 works with E3372 perfectly. Please consider using sms-tool to add support for SMS to the GL-iNet UI 