I'm not entirely sure if the vivoactive 5 (30.40 mm, Wi-Fi only) is compatible with a Polar heart rate belt. The information available to me does not fully cover this specific combination.

Garmin devices usually support ANT+ or Bluetooth sensors. Polar heart rate belts also use these technologies, but compatibility between different manufacturers is not always guaranteed. Garmin recommends using original accessories to ensure full functionality.

I would recommend contacting Garmin support or consulting the official vivoactive 5 documentation to ensure compatibility with Polar heart rate belts.