Polar H10 does not show up on any activity in the app. It is connected and it is visible as sensor last used in 90 days on web app. During activity on the watch when I check sensors it says that my Polar H10 is connected - I can even take off my watch and activity still detects heart rate without problems therefore h10 is connected and running.
I have found forums.garmin.com/.../polar-h10-is-connected-but-doesn-t-appear-on-garmin-connect-when-recording-an-activity and it indeed is in the webapp on the activity in network request but why it does not show up in the app when viewing the activity? It should say at least external sensor or something.
In addition to that it should be displayed under "... more" button -> Garmin Devices -> Sensors section.
Here is a screenshot of "devices" from webapp (Heart Rate Monitor 23123):