It would be convenient to have a phone app which could control smart trainers via TrainingPeaks.

There are multiple mobile, and desktop, apps that will import structured workouts from TrainingPeaks for controlling smart trainers. These are some of our favorites:
https://www.trainingpeaks.com/blog/choosing-the-right-indoor-cycling-app/
There is an app for Garmin devices that import structured workouts from TrainingPeaks. The newer Garmin models will control smart trainers that support the ANT+ FE-C protocol.
https://apps.garmin.com/en-US/apps/4b834c4a-96a9-422a-a945-08cea2ee571d
The Wahoo ELEMNT and ELEMNT Bolt are also compatible with structures workouts from TrainingPeaks, and will control the Wahoo KICKR.
https://www.trainingpeaks.com/blog/get-your-daily-workouts-instantly-on-your-wahoo-elemnt-or-elemnt-bolt/
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Peter Cole commented
I have a WattBike but use various other trainer bikes too, or my bike on the turbo. The new structured workouts feature is good but does not support any of my devices. It would be good if TrainingPeaks could 'run' the workout, and then pop a notification onto the screen at the start of each interval with a text description to say what to do next.