I have a working library for initializing AT-command compatible modems for TCP/IP over cellular.
Protocols: TCP and UDP (both work)
Mode: Transparent Bridge
Rate: 57600 baud
GCS tested: MP and QGCS
HW: APM1 tested, (APM2 testing soon)
Modems Tested: DroneCell (SIM900D module) and Arduino GSM Shield (SIM900 module)