Thank you for the information.
I suggest using option driver,
echo "0e8d 7127 ff" > /sys/bus/usb-serial/drivers/option1/new_id
Your AT command port should be If#= 6, in the absence of other tty's it will be mapped to ttyUSB4, but with properly patched driver it should be ttyUSB3, as currently you have your ADB interface (If#= 5) occupied by the serial driver as well.
Typically in such setup the gateway IP is not needed, as well as the correct netmask. You can use a device like wwan0 as your default gateway.
Can you confirm that DHCP does not work with this modem, so manual address configuration is the only option?
If you have a chance, please check:
AT+GTUSBMODE?
AT+GTUSBMODE=?
It should be 40 and 41, but who knows 