hrm dual vivofit 4 and iphone 12

You have to pair the HRM strap directly to your watch. Look up the manual for the term "sensor pairing", or go to the menu Sensors & Accessories on the watch and pair the HRM there (you must wear it, when pairing!).

trux said:

You have to pair the HRM strap directly to your watch.

The vivofit 4 does not support external sensors.

OK, I see. Sorry, forgot to check the specs., in that case it explains it. Unfortunately, Garmin will certainly not retrofit the device with this functionality. A watch supporting external sensors is needed. 

But i dont understand, if i connect via bluetood hrm dual with my iphone and it works with healt app, why dont work with garmin connect??

I do not have any experience with HRM-Dual, but HRM-Pro, when paired with Garmin Connect, can indeed deliver HR data to the all-day stats, when the watch (or band) is not worn (or the OHR sensor is disabled). I guess (but do not know for sure) that it should work in the same way with HRM-Dual. However, you can observe the HR in Garmin Connect in real-time only when an activity is started, and that is possible only when the strap is paired with a watch, where you can start the activity, since that is not possible with the strap alone.

I tried with the sensor in my chest to see my HR on real time working in the app of garmin conect but dont appear anything only says "add data manualy" only i can see my HR in the healt app for apple and if i want to connect another device, dont appear in the list of devices. i have a vivofit 4 conected 

Yes, that's what I wrote. HRM-Dual is not designed as a standalone device to be used as the Primary Wearable Device and the main source of data for Garmin Connect. As I wrote, you need a watch able to use the strap as a sensor. Still, you can use 3rd party apps, like you already noticed. Also for example Strava, and many other phone apps can read the HR from the strap directly.