@T7H Tried all of that already, router in bridgemode, use as access point etc etc... nothing worked until I changed the cable --> 300Mbit!

I used to work fine with my old Ziggo modem, together with 3 Deco M9 devices set up as an access point. Now I had to change my old Ziggo Modem to a new Ziggo ConnectBox, so I thought it would be...