Connect Garmin to iPhone (w Phone acting as the bridge)

I'm trying out this app, which takes data from HR / Cadence etc and then rebroadcast it to Zwift

RunCline - https://apps.apple.com/us/app/runcline/id1450214714

When I try to connect my Garmin Fenix 5X+ (which supports BT), It is not able to find the virtual HR/Cad data. Does anyone have any clue why this is?

Is it something from Garmin's end or Apple's?