They don’t work together. Which is a pity for a system so expensive – but also not a surprise given the overall poor user experience (UI, ease of use) with the product.
The solution: Sabrent Wi-Fi Audio Receiver which is out of stock on their website but may be found reconditionned fe. on Amazon, costs 20 euros and fills the gap.
Bonus point: can be USB alimented by the unused USB port on the back of the system.
Finally, I moved from a Garmin Forerunner 225 to a Vivosmart HR+. Mainly this is because the Vivosmart is smaller, lighter and has a GPS and also because, honestly, I don’t need all of the Forerunner features. Moreover, the Forerunner doesn’t mount on MacOS so one can’t upgrade firmware, transfer calendars and training plans and so on.
The vivosmart is a good fit, but there are some not-so-minor caveats:
GPS is slow, and if you decide to give up, you don’t have a choice besides a indoor activity (with the Forerunner you could run an activity with a poor GPS tracking anyway).
You cannot manually mark laps. There is only one button, and is for start/stop.
From a tech perspective, you have all the sensors and the analytics capabilities of a high level device (accelerometer and so on). The software is amazing, and tracking an outdoor run without GPS on a known path provided an excellent result (equivalent to a GPS enabled tracking).
Finally, the vivosmart is a good deal for me (YMMV), especially because I manage to train intervals on track.