Hello @Joost_de_Bonth,
Can you please specify the model/manufacturer of your Router that you are using? Can you confirm that your remotes are connected to the Router and not to a Range Extender of sorts?
I would try connecting your Core and your Macbook remote directly via Ethernet to the Router to see if the same issue exists then, if it doesn’t then it may indicate Wifi signal problems. If it does, it may indicate some configuration has to be done on the Router.
I would double check Multicast Settings on the Router and IGMP settings, multicast has to be on for device discovery and sometimes toggling IGMP settings helps resolve discovery issues. You may also want to set up firewall exceptions on your Router if the firewall could be the issue here, for the specific ports I direct you to Brian’s post here:
Thanks,
Noris