Don't go with smart TRV's, just get weather comp and don't look back. Vsmart or VCF700 depending on the rest of what you have. Once you have weather comp you wont need TRV's.
I have the same boiler and added VRC700 and VR70 wiring centre, I squeezed the NTC in behind the original cylinder stat.
This setup will do exactly what you want. We have two heating zones UFH and Rads and PDHW. The weather-compensated heating hasn't got over 50C yet even when we were -1 a few...
Mine was sketchy yesterday, often had the updating servers message, this morning took a while to connect and sync but got there in the end after logging out and back in.
They do seem to run updates each weekend, at least the last 3 weeks that I've had it.
Glad you are up and running, shame it was new parts but that is what it is, just annoys me when tech support point to things like wif when it needs to do a radio handshake first..... I do security systems so deal with radio alarms and wifi and networking pretty often, so few support people that...
Any joy today, did you get hold of Vaillant? I don't know much about this specific system but it must be a handshake issue between the thermostat and the receiver, I think the lack of internet yesterday must be a red herring as the system must be able to get up and running without internet...
I'd try uninstalling and reinstalling the app on your phone, I can't see it being Vaillant's end with servers still down, the thermostat pairs locally first I would guess as they are 686mhz radio to the receiver so that should give some control back.
Reboot the gateway and or your router, leave the router off a few minutes to clear any cache if so. Mine's not the vsmart system by the way, its the VCF700 platform. I assume yours is a combi boiler?