I’ve got it to work today!
The solution was changing this
[ipv6]
addr-gen-mode=stable-privacy
to this
[ipv6]
addr-gen-mode=eui64
in the file described here https://github.com/home-assistant/hassos/blob/712dcf9f74497eef6a75ed1e76f8ae8817ae3ee6/Documentation/network.md.
The meaning of the settings described above can be found here: https://developer.gnome.org/libnm/stable/NMSettingIP6Config.html#NMSettingIP6ConfigAddrGenMode.
I hope this helps someone else as well! 