If it is anything like my modem, it may be /dev/ttyHS0, /dev/ttyHS1, etc.
I would try ls /dev/ttyHS* and see what is listed, then make a symbolic link pointing to the correct location:
su <enter password>
ln -s /dev/ttyHS /dev/ttyHS0
That may be all there is to it. Then again, 3g modems seem to defy logic.