@TriathlonTom the modern way is to use an ANT+ to Bluetooth bridge. If I was starting over, would use this one:
4iiii Shop - USA/World: Heart Rate Monitors: Viiiiva combination Heart Rate Monitor, ANT+ bridge, and data recorder
These guys make a stand-alone unit, but that seems clunky in the gym:
https://npe-inc.com
I have an Wahoo ANT+ adapter, it uses the older 30-pin connector and that works for me in the gym with Stages bikes:
