Open a terminal prompt and, with the modem unplugged:
install the usb-modeswitch package
# sudo apt-get install usb-modeswitch
After this completes, reboot the Pi and then log in again and open a terminal session.
Plug the modem in, leave it for a second or two for it to be recognised and then do
# lsusb
Hopefully, the output will now indicate the the modem has been recognised. Display the network interface list:
# ifconfig -a