Hi @Ben_de_Groot and welcome to the forums! I haven’t been able to connect with my Garmin dual HRM since the update at the end of October. Here is a link to the thread on this topic and the ideas that several people have had. It seems that some people are able to find a workaround and some aren’t : https://forums.zwift.com/t/garmin-hrm-no-signal-solved/209400/140
Good luck, and if you find a solution, please post it!