Have a read of this:
I have the DH mentioned in this thread. I use it as follows:
I can change the thermostat temp using SmartThings which in turn uses the DH, that then sets the temp on the thermostat (ASR-ZW) which in turn controls the switch (HRT4-ZW).
The question is can you connect the HRT4-ZW directly to ST and then turn it on and off? I’m unsure but I do have a spare one. If no one else has any other ideas I’ll try and have a look for you at the weekend.
Keep your eye on eBay. They sometimes go for next to nothing.