I connected the four pins in Black-White-Red-Green order. The text shows up correctly, but when I want to “Hit any key to stop autoboot” nothing happens and the countdown continuous normally.
I’ve tried using both picocom and screen.
I’ve got a CZ11N20 model. I’ve heard hat some of these don’t have a serial ports. But those don’t have the pins at all right?
Hit any key to stop autoboot … 
…
https://doc.turris.cz/doc/en/howto/omnia_booting_from_external_storage
It I remember from my story mSATA SSD system disk … depends on the moment (hit Enter) and it must be very soon. 
The first time it will not work and the second time also not, and the third time not yet, … 
In the serial console, when powering it up, try to hit enter repeatedly as fast as you can till you manage to interrupt U-Boot bootloader and you get ‘⇒’ prompt. Remember how you did it, we will need it later on again.
In this prompt enter run rescueboot and wait for rescue mode to boot up. Once you see the prompt, we can start preparing the disk. At this moment, you probably have two disk devices available - sda and sdb. Most likely your USB/SSD is sda and USB with medkit is sdb. You can check it by entering:
Does the device echo or do I need to turn it on in the terminal-emulator?
I do arrive at the => prompt because there’s no kernel image. Typing “run rescueboot” does not do anything.
Try … rescueboot run
My way …
Is the echo from your terminal emulator or from the omnia?
Found out that I had the pins plugged in the wrong way
Just swap Tx and Rx on one side ?