Octoprint serial connection via GPIO pins, how?

I'm trying to connect Octoprint running on RPI3 via GPIO pins to Prusa MK3 (Einsy Rambo 1.1b), not via USB cable.

I have enabled RPI port via Prusa menues, which should enable TX and RX pins.

On RPI3 I have run raspi-config and enabled hardware serial port but disabled linux console via serial.

I have connected TX, RX and GND pins, but I still can't connect them via Octoprint web UI.

I now have /dev/ttyserial0 and serial1:
serial0 -> ttyS0
serial1 -> ttyAMA0

So should I use ttyS0 or ttyAMA0 ?

Any other suggestions?

I tried reversing TX/RX pins but still serial is not connecting.

One side receives what the other side transmits so yes, Tx/Rx need to be crossed
Did you connect GND to GND?
Does the kernel see the connection? (dmesg)

This may be helpful: Simple 3 wire serial connection from PI3 to SKR V1.3 board

ttyS0 is the GPIO serial port. You'll also need to enable it via raspi-config. Do not enable console over serial.