antst wrote:I tried to enable ventilation, it was a first thing I did

Didn't;t have luck. But maybe then I have to configure override for ventilation function on OTGW, so thermostat thinks that there is support on other side.
Indeed, you have to use the command
SR to set a correct response to the thermostat. You have to alter the slave configuration if I remember correctly, that's Data-id 3 or 74. To check out the bitfields of the two data-id's to determine the correct data-id and response, turn on
Include details of bit fields in OTmonitor->Configuration->Logging.
SR=Data-ID:data
Set Response - Configure a response to send back to the thermostat instead of the response produced by the boiler. The data argument is either one or two bytes separated by a comma.
Example: SR=18:1,205, SR=70:14