hc-12 and Raspberry Zero - not getting /dev/ttyS0

[John Public]

I am trying to test HC-12 on a Raspberry Zero. I am following the example at
http://dev.drun.net/2017/05/10/reading- ... -domoticz/

I've connected wires as in the picture there and then test if /dev/ttyS0 is available. It is not.
I tested both 3.3V and 5V pins on RPi Zero.
I also tested on an OrangePi PC and there /dev/ttyS0 is available as expected.

I also checked from raspi-config that my serial interface is available. It is.
I tested on two different HC12s. Both worked on OPPC but not on the Zero.
Just to make sure that my pins are working I tested a LED from each suitable pin in the Raspberry Zero.

What could be wrong or waht have I missed?