I've just got my Coros arm HRM, connected it, seems to work but I don't see it listed on my activities.
I turned off the wrist heart rate monitor so it's working, no breaks in the graph.
It used to be below the watch photo:

seems to work but I don't see it listed on my activities.
Only supported sensors transmitting the enhanced device info, such as the HRM-Pro, or Speed Sensor 2, etc., are listed under …
Here is the list with sensors that support this feature:
Why Is My Paired Sensor Not Showing on My Activity Details Page?
seems to work but I don't see it listed on my activities.
Only supported sensors transmitting the enhanced device info, such as the HRM-Pro, or Speed Sensor 2, etc., are listed under the device status. For other sensors you'd need to use browser's DevTools, where it is possible to look up all connected sensors.