Just to summarize what the authors steps for using T99W175 on any m.2 to USB conversion board including GL-M2:
- Modify the T99W175 according to this Russian website - https://4pda.to/forum/index.php?showtopic=1086317#Spoil-129097688-9
basically, soldering the two connectors; then use a tap to tap off the 2nd most right pin.
This will make T99W175 in USB mode. This however, does not mean that T99W175 has 'AT' command interface enabled (enable ttyUSB) (This requires USB mode enable).
The reason 'AT' command interface requires to be enable, because it is how ModemManager talks to T99W175, including command for initiating connection with mobile data provider.
This is the genius of this post comes in.
2. enable ttyUSB.
Following the instruction of this post. The only part need to be pay attention is the generic driver location.
So: echo "05c6 90d5" > /sys/bus/usb-serial/drivers/option1/new_id Tis line of code might not be suitable for everyone, on arch the command should be: echo "05c6 90d5" > /sys/bus/usb-serial/drivers/generic/new_id
After this, ttyUSB will be enabled, this in turn will enable 'ModemManager' to execute its command that requires 'AT' level interface.
Hope this helps.