@harmonisch , Welcome to the community!
A new Conenct box will not fix the cause. Have you ever tried it with a WIFI Booster? Please contact our technical support at 0800 66 88 66 so they can check with you.
@harmonisch , Please name your WIFI networks differently again. For example, add _5GHz to the 5GHz WIFI. Also remove or switch off your Netgear PLW1000v2.
If it still doesn't work, contact our technical support at 0800 66 88 66.
@harmonisch , Please contact our support so they can check if your modem can be replaced. I also see a D-Link device on LAN 2. Is that a WIFI router? What happens when you unplug it, does everything work?