I do not see controlable trainer as an option on the iPhone. It appears on the Mac. What is the problem?
Make sure it is not on the incompatible device list. Update all the firmware you can if it is not.
Another possible example:
Look for BTLE or Ant+ on the added devices you are using. If you are setup for Ant+ and a BTLE sensor is added somehow on the phone, “forget” it then add the proper one.