After using the thermostat and four radiator valves for about two weeks, the AI Assist trial period expired.
It then turned out that the only feature I bought this system for only works with a subscription!
If it's warm in the living room but not warm enough in the bathroom, the bathroom valve has to tell the thermostat to turn on the boiler, even though the thermostat itself is already warm enough.
I then complained to the TADO helpdesk, and the only reason they've enabled the feature for me now is because I bought the thermostat before October 1st.
So I suspect they only hid this feature behind the subscription for two weeks, forcing everyone who buys the system from now on to sign up for a subscription. Without this feature, the smart valves are nothing more than incredibly expensive thermostatic valves.
Hopefully, someone will now come up with an alternative, for example via Home Assistant or other home automation software, so that it can work outside of the TADO app.