Losing HRM signal while zwifting

Hi all, I hope someone can help me out. I have some trouble with my HR monitoring while zwifting. I’ve a Garmin fenix 5 sportwatch connected to Zwift via a Cycplus ANT + dongle inserted in my USB extension in my Macbook Air. My Tacx Flux Smart is connected to Zwift via Bluetooth. When I click the search button to pair my HRM/garmin it shows up and connects with no trouble. The thing is that while i’m broadcasting my HR to Zwift while doing a workout it sometimes losses the signal and than after a while it connects again. Because of this my heart rate monitoring is inadequate and unreliable.
My watch is within a meter from the ANT+ dongle in my Macbook. I’ve tried to connect my Tacx also with the dongle and the same thing happens: the signal gets lost sometimes. That’s why I’ve decided to connect my Tacx via Bluetooth. Does somebody know what’s the cause of this and how to fix it?