How to simply turn heating on/off with VRT 340f thermostat?
I reviewed the manual for your VRT 340f thermostat online. It does not appear that this unit is intended for use in a simple on/off on demand mode. From the description in the manual it appears that unit is intended to be operated on a schedule. 

Your most logical choice may be to replace the thermostat with a different model that offers the simple level of control that you desire. Since this thermostat communicates with the heater unit using a radio link your choices of replacement unit are going to be limited to what is compatible with the heater.