Dongles are a pain in the arse!
The problem is that they do not really follow much of a standard.
The 3G dongles we support in the FireBrick all appear as a "file system" initially so that one can install windows drivers, and then magically change to some sort of serial interface.
The way the change varies - some "eject CD" on the file system, and some do something else. At least one does a USB profile setting (the right way to do it, arguably) and one is just strange.
For almost all you then end up with what looks at a USB level like a serial port. What you do then is talk old fashioned Hayes modem dialling and set up commands, and then serial PPP coding. It really is that daft.
The dongle is faking all of this, it actually talks a "PDP context" on the air, having pretended to talk LCP and PAP/CHAP auth and even IPCP on the serial interface. Eventually it connects and passes IP packets over PPP over serial (with escaping characters).
The new 4G dongles appear to be different - they seem to appear as an Ethernet NIC.
But we have yet to get to the bottom of it - do they talk faked PPPoE or what. That is the next challenge. We'll see.