I have created a script and a luci-proto app to the FM350-GL.
It support dual stack and, I think, authentication works as well. The script re-connects on network disconnections. Received SMS are stored in /tmp/sms/rx folder.
Fell free to try.
More info and the script: https://github.com/mrhaav/openwrt/tree/master/atc/fib-fm350_gl
luci-proto-atc: https://github.com/mrhaav/openwrt/tree/master/atc
Edit: The hotplug script for the Option, or Generic, USB driver didn´t work as expected. Now it´s corrected.