EX6150v2 Range Extender Kills Ports on Router

I have a Router (DHCP on) and WiFi Access Points (Netgear) in two different sides of the house. To get better connectivity in the basement I got a Netgear Range Extender (EX6150v2/AC1200) and did the setup. Everything worked fine. But there was no Internet connection available. And when I disconnected the Range Extender, I found that the port on the Router was dead! I had to plug-in the WiFi Access Point in another port to get it working again. This happened two times already.

What is wrong? How could the ports on the Router go dead on installing the Range Extender?