@Bernhard_Sailer. The Atag-One is not what is meant with “Opentherm gateway”. That is a dedicated piece of hardware sitting between an Opentherm compatible thermostat and the boiler. With the Atag-One you do not need that, because it has its own wifi interface. that is what you use with the Atag-One api from Rob Juurlink.
The Opentherm gateway was designed for themostat without this possibility and are often home built bij electronics hobbiists.
Gert