On the latest version of Oreo the permissions issue @NIKO described above has been resolved and the option is no longer greyed out, and I’ve been able to re-enable the “Use ANT hardware” toggle. This fixed it for me, so looks like it may be an OS version waiting hame.
For reference, I’m a OnePlus user and my phone on Oxygen 9.0.5 has the issue and on Oxygen 9.5.11 it is fixed.