The Call for heat feature can only be activated via the web app : https://manager.lightwaverf.com/
You should not pair TRVs to a Thermostat. You simply pair the Thermostat to the Boiler Switch, add the Thermostat and all your TRVs to the app.
Now you Activate the call for heat, choose the Thermostat as your parent, and your TRVs as Children, so if a child is cold and needs heat then it will ask the Thermostat (parent ) for heat, the Thermostat then will send a command to the boiler switch to turn the heating ON.
The Call for heat follows each TRV schedule. SO you must set schedules for each of your TRVs based on when you want them to call for heat.
Call for Heat will set the temperature of your parent controller to either 40C or 0C depending on whether heat is desired or not desired. If your heating was set to 40C after your power cut, then one of the child devices must've been under temperature and needed heat. These temperatures are chosen as maximum and minimum temperatures to ensure the heating will go on or go off as per the childs requirements.
If you would not want 40C and 0C to be used, then you would need to turn Call for Heat off.