Ok, so I have contacted Essent, and I got reply.
I need to deregister here:
https://mijn.icy.nl/nl/e_thermostat_deregistrations/new

Which costs mee 15 euros. Did you guys have this to?

OK, so I got one from Marktplaats.

The previous owner has to de register the thermostat, before I can use it. (So tip for the next one: You can prepare the seller for this)

There is an USB connection om the hub, did you try this to communicate with the hub itself?

*Edit*

Ok, USB is power only I ...

Hi All,

I currently have an Intergas Kobikompakt HRE28/24.
I am controlling it with a Honeywell Chronotherm IV On/Off.
I know my heater supports Modulation/Opentherm, so I would like to replace the thermostat with a better option.

I do have a domoticz system running, VPN etc so I do not depend on ...