I made it work with an Ant+ dongle and and OTG cable on a Moto G (Garmin Uploader, rather than Trainerroad, but same principle) - bit of a ballache to remember where I put them, but easier than a Windows computer just for Garmin nonsense.
Something like this, where 'OTG device' actually plugs into your phone. With possible slight variations if your phone/external battery aren't USB Micro B, or your dongle is not USB A.
I made it work with an Ant+ dongle and and OTG cable on a Moto G (Garmin Uploader, rather than Trainerroad, but same principle) - bit of a ballache to remember where I put them, but easier than a Windows computer just for Garmin nonsense.