Thanks for your response.
There is a second coax outlet inside the bedroom, which looks just the same. I didn't try it out tho.
Sidenote: there was originally a splitter on the outlet I use. I removed it because it was even worse before, with a lot of ping strikes.
I read elsewhere that there shouldn't be any PostRS errors, and here I see there's a lot. What does that mean?
Regarding DHCP: it's already on 86400.
Regarding Firewall: done.
There's also someone from the ziggo support that reseted the routeur.
I'm gonna try with these options and give an update whether that works or not. Thanks!