This is the working example for STM32F407 and Quectel UC20 3G module as PPPoS adapter.
This was tested with Quectel UC20 3G modem with UART connection
The board from Olimex STM32-E407 which STM32F407 based
https://www.olimex.com/Products/ARM/ST/STM32-E407/open-source-hardware