OK, I have tested my OTGW with ESPEasy unit for WIFI communication, with the Remeha Tzerra 28c plus, and eTwist thermostat (Room Unit).
Although the Tzerra is controlled with OpenTherm, the eTwist is not. To make it work, Remeha provides a small white box, called E-twist gateway.
The wire between the Room Unit and the gateway is using R-bus. The wire from the gateway to the Tzerra is OpenTherm.
So: I installed my own OTGW between the Remeha gateway and the Remeha Tzerra. Activated the unit in Domoticz, and I could see the usual info coming through in the log. At first a lot of hex data, but after a little while the info made sense and I could recognize several OTGW values.
Sounds promising, but wait. I tried to change the room setpoint. My device in Domoticz changed to the new temp, and in the log I could see the TT: command going out.
Unfortunately the E-Twist room unit did not pick anything up.
Also, the Flame On signal was coming through, but it was not reliable.
My conclusion: the Remeha Gateway unit does not like working with the OTGW.
As another test, I took the unit back to my home (the Tzerra is installed in my summer home).
At home I use a Remeha Calenta, with an iSense thermostat. Both are pure OpenTherm. I have an OTGW installed between the boiler and the room unit, works great.
So I decided to put the second OTGW between the first OTGW and the thermostat. This did not work well, similar results as the test with the Tzerra.
Conclusion: use only one Gateway between the RU and the boiler. Gateways switch in serial with another gateway don't work well...
In my case it is not a problem: with the E-Twist I can control the unit from my Android phone, so I don't really need an OTGW there.