This page will explain some connection issues.
Cannot connect trainer
Bluetooth connection to other app:
Please check that all applications are closed on the background. When the trainer is still connected to another app, it will not connect to the Cycling app.
Select the Trainer Toggle (do not slide):
It is important that you do not slide the button to connect the trainer, Press on Trainer button and all the toggles become blue. When you press stop search, the grey icons on top will illuminate.