GPRS Shield V2.0 bought from Exp-Tech

Hej there,
I’ve some problems with my Shield mentioned above. It is not possible any more to transmit data to it via the UART TX-RX.
The messages from the modem can be read clearly in the terminal (HW-Serial on Arduino Mega). Tested different cables and µCs already, baudrate is now at 38400 bps.

Any suggestions? Can I maybe reset the board to factory defaults?
Regards, Basti