Simple upstairs and downstairs thermostat setup doesn't work

I have a wired thermostat downstairs in my house, and a wireless thermostat upstairs. I also have hot water control. (I.e. a starter kit with hot water control plus a smart thermostat add-on)

If the upstairs thermostat demands heat and the downstairs thermostat does not, the upstairs radiators get hot as expected, and the downstairs radiators stay cold as expected. However, if the upstairs thermostat is not demanding heat, but the downstairs thermostat is, no radiators in the house get hot. I can only heat my downstairs radiators by having both upstairs and downstairs thermostats demanding heat.

Can anyone help with this? It's baffling and very frustrating