Wrong newline character over serial port (CR instead of LF)
Solution 1:
If using picocom, it's a matter of mapping the serial output from CR to CR+LF.
Example: picocom /dev/ttyS0 --baud 19200 --omap crcrlf --echo
Actually able to send commands to the device now!
Solution 2:
Please ensure you have set the ttyUSBx port in raw mode. Otherwise there can be some character replacements e.g. \r with \n and vice versa on receiver side.