No response from serial port within addon


I have a SIM800 module on my rp3 B+ that works if I use Stretch Lite. Only thing needed to do was to add the pi3-disable-bt overlay and disable serial login.
Using python and pyserial I could open /dev/ttyAMA0 and send AT commands.

I now installed, added the pi3-disable-bt and disabled [email protected] via host ssh and created an addon with auto_uart to true. I communicate with the addon through stdin.

I can open a serial connection to /dev/ttyAMA0 within the addon but I am not getting any response back.

Any idea what I am doing wrong?

Thank you!