Hello Jan_Stes,
I have moved your request to the support category of the forum that automatically notifies the @support team.
Please make sure to describe your network configuration/topology, including any networking hardware currently in use, so they have a clear understanding of how your devices are connected.
I saw this old thread here, have you tried by now to set different DNS servers for your router?
That might very well improve your situation, please have a look:
How did you configure your router to get ARC working?
Can you please provide a screenshot like this one (of my own setting)?
Thanks!