Sierra Wireless BX3105 UART not responding via header pins

Hi there,
we have no issues to communicate with our BX3105 Board via UART USB. Unfortunately it is not responding to AT commands given via the header pins, although the jumper for UART source select is set to header. Also the baud rate is set to 9600 at both boards, boards have common ground, connection is RX-TX;TX-RX.

We want to connect the wireless module with a NXP microcontroller, so we are sending AT commands (e.g. ‘ATI’) via the controller (we measured the pdm signal, it is fine), but there is no response from the wireless board… we also checked it with another controller (ARDUINO) but still no response…