Hello both.
Not sure if Riddy ever found out why, but I did the exact same thing, and after (a LOT) of back and forth with Audi connect, I was informed that my 2018 A5 (or any others before 2020) were not fitted with the auxiliary heater required to allow remote heating.
This was confusing, since on the spec list on my myaudi app for the car, it clearly lists auxiliary electric heater, but when speaking to the Audi guy on the phone (pretty certain he was, in fact, a robot....) I told him that the app states the car must be fitted with the aux heater, and the spec says it is, but he simply kept repeating that the car was not compatible.
Eventually after speaking to someone else, they advised that apparently this was only started to be built in to the cars to be able to remotely operate the heater from 2020 onwards, so A5's from 2018, 2019, and some 2020 will not be able to access this feature.
Hope this helps.