Hi guys,
I am having the same problem (no heart rate with external strap) with my Polar H7 (Bluetooth, no ANT+) sensor. I wrote a thread in this forum with the steps I took and tested. https://forums.garmin.com/sports-fitness/healthandwellness/f/vivoactive-3-3-music/212622/polar-h7-only-working-with-garmin-apps
So this might be a problem of not having ANT+ feature?
Any news on this?