There is a button marked "heating" on the unit which if pressed will set all rooms to 21C and heat for 4 hours.
Useful feature. But... it's software. There's no reason that should work perfectly, especially with a borked system, whilst other parts of the same product 'might' have issues.
Once upon a time I was monitoring some piece of comms kit and treating the signal LED as 100% honest, as if it were hard wired into the cable coming out the back. Took a significant effort to consciously realise the LED could fib - not only did I know it was connected only to a CPU IO pin, but I'd programmed the thing myself. Strikes me that the 'heating' button is similar - it fools you into thinking it cannot be wrong and will always do exactly what it's supposed to regardless of everything else going on.
I doubt you'd find any thermostat that isn't at least somewhat software controlled, except perhaps the most basic thermo-metallic contact unit. But even then your boiler probably has a microcontroller in it... The internet connection on the unit is the riskiest part, given it depends on external infrastructure that I can't control. All that the 'heating' button needs to do is turn on the boiler (the relay clicks immediately when I press it, though I use OpenTherm for 'reasons') and send some Zigbee packets out to get the thermostats running. It should be pretty reliable, at least as reliable as a gas boiler itself is.
In the very worst case, I still have the old thermostat wired in parallel as a frost stat set to 5C. If it all goes to pot, I turn that on and then either remove the TRV's (100% heat) or set them manually via Zigbee commands.
There was a reason that when I was looking at smart heating I went for an independent system. While it uses Zigbee like the rest of my smart home stuff, the TRVs talk to the Wiser hub, and the hub manages all temperature control. My RasPi can send heating requests - I use it to do neat things like turn off the heating in my home office when away from home - but ultimately the white box next to the boiler does all the hard work of getting those rooms to the right temperature. The Wi-Fi interface even works if the internet cloud goes down as the Pi accesses it via the local network. I almost never use the cloud interface anyway.
I have Homematic IP wireless system for underfloor heating. Although ecosystem has many more different smart home devices some which I may start using later and is compatible with some other manufacturers. It can be set up in 4 different ways. One is pairing room thermostats directly to underfloor valve controller or radiator thermostat but it's the least versatile and with no smart garbage whatsoever. Second way which I'm currently using is connecting all of the devices to a $50 ethernet or wifi access point which allows setting things through the app. App works through the cloud, however otherwise system is autonomous and without internet connection you still can set temperature on thermostats, light switches will still work, etc. App does not require any registration at all, you just scan a QR code on access point or enter code printed on it manually. Third way is using CCU3 central control unit instead of access point which works locally and you depend on nobody. Everything happens through web UI instead of the app but it costs around $190 which is almost 4x more expensive than access point. Alexa and other voice control can be added too if you want to deal with that garbage.
Or 4th sort of unofficial way (interface HW is official, software isn't) is using Raspberry Pi or other device with a hat or dongle and running RaspberryMatic https://raspberrymatic.de/
The downside is that Homematic is almost entirely Germany oriented so good luck finding any decent info about it on anything but German.
Sounds similar enough to Wiser given you can just go to 192.168.x.x and once you know the "JSON secret key" (which is retrieved by pressing a button on the unit and within 30s going to the right page) then you can get local access. It's almost as if they built it to be tweaked with by home automation nuts. Thankfully someone has done the hard work already and built an integration into Home Assistant, and occasionally the engineering manager for the company posts on the HA forum with bug fixes and suggestions, so it's informally supported, which is nice to see. That integration was a major reason for me purchasing it: other products didn't have it, or only had it via a cloud 'app', eww.
I found that the kitchen had a hydronic radiator built in (I first thought it was an electric kick fan but noticed the '80 watts' label on it). So I set it up so it runs off a Zigbee smart plug, and when the boiler flow temperature exceeds a certain temperature and the kitchen temperature sensor is below another threshold, the hydronic radiator turns on, around normal dinner/cooking time. It's a little crude, but it keeps the kitchen warm in winter without wasting the heat for the rest of the day.
Hot day today, so the heating is all off, and yes I think I want air con ... I just need to figure out how to automate that next. Most mini splits seem to be wi-fi controlled, annoyingly, so I will probably figure out a way to do it via the infrared control.