The thermostat has to use permitted radio frequencies, so to make sure it only works with the right signal, it also sends a tone superimposed onto the frequency to identify it as the signal which should be used, it is specially designed not to respond to next doors thermostat, or other items using radio frequencies.
To ensure it works, it needs two way coms of some sort, i.e. turn on boiler, and a reply OK turning on boiler, there are cheaper methods, like if no signal received in 30 minutes assume battery is flat on thermostat and turn off boiler.
But in theroy at least it should not matter, if the wall thermostat gets stuck on, the TRV (thermostatic radiator valve) will turn off anyway, so all it means is the boiler will cycle on/off. However with most TRV's having a setting *123456 it is a bit hit an miss if set correctly, and even more hit and miss as to if lock shield valve set correctly which stops an over shoot.
However the TRV heads can be replaced for the electronic type, and one will have ºC and an app on your phone.
I have a Salus ST620 thermostat attached to a Worcester boiler that’s not working properly
What is it doing wrong? It may be a TRV and the wall thermostat are fighting each other, rather than working in harmony. A TRV and wall thermostat in the same room can be set to work together, but it takes some work and time to set them to work together, far easier if the TRV is linked to the wall thermostat.
The standard instructions years ago before we have modulating boilers was not to have a TRV in same room as wall thermostat, and the wall thermostat should be placed on a lower floor (as heat raises) in a room kept cool (as we don't want heating to run when it is likely to be a warm day) with no alternative heating, this includes heat from the sun through the windows, and no outside doors. I don't know about your house, but my house there is no such room, so only way is in my case use the hall, and have a TRV which will allow rapid reheating of the area if a door is opened, but will reduce the heating so other rooms also heat up.
However this worked A1 with mothers house, but not with this house, so it does seem one system does not suit all. The EvoHome is very good and likely to work with most homes, the Drayton Wiser is also very clever with learning TRV heads, but Hive is cheaper, however to get good control you are looking for a wall thermostat which links to the TRV heads, unless living in an open plan house. Tado I am not so sure about as there are a few thermostats in the range, and not all are linked, but Worcester Bosch do their own thermostats as well, but last time looked, although connect to ebus so modulate the boiler better, they like Nest don't connect to TRV heads so are rubbish really.
But it would be better if you say what is wrong with your existing system, and also how much you are prepared to pay, I work with the near enough engineering principle, there is no point paying 100 of pounds if you can get a system good enough for your needs for £50, so the guides may say do this and that, but some times a simple on/off thermostat is enough.