This should be easy to fix on Raspbian provided it has a USB serial driver we can use. Try option first:
modprobe option
echo 1199 90xx > /sys/bus/usb-serial/drivers/option1/new_id
You’ll have to replace the “1199 90xx” with the modems current PID and VID. If that works, then youl’ll have a number of ttyUSBx devices and just have to try which one is the AT serial function. I assume you know how to go from there since you’ve already changed the PID.
If you got a write error, then your Raspbian didn’t have the option driver. If so, then please show us the output of
ls -l /lib/modules/uname -r/kernel/drivers/usb/serial
so we can see if there is another usable driver.