Step-by-step instructions to build custom firmware, flash the device, and final setup steps to work with or without Home Assistant:
https://thehelpfulidiot.com/a-wired-sonoff-zigbee-alternative
kerimcelik, I need to add two separate houses to my HA-system, so that is not an option.
francisp, I will try that, and will be back. Since that is different version board (V1.2, I have V1.4), I bet it will not work.
Lets see what happens, I’ll be back! 
I suggest you just start from here:
The device is now set up to use with Zigbee2Tasmota. For Home Assistant, there are a few more steps.
That did the trick! For future people who are reading this, the answer is to compile your own with this guide (thanks francisp): A Wired Sonoff Zigbee Alternative – The Helpful Idiot
Finally add to console:
backlog rule1 on system#boot do TCPStart 8888 endon ; rule1 1 ; TCPStart 8888
and now it will listen to port 8888, even after reboot 