After not switching on my Xbox One for a few months, it wouldn’t connected to my WiFi. It kept saying “Can’t connect to DHCP Server”. Rebooting the router, and forgetting the WiFi network made no difference. In the end, the way to fix it was to go into my router’s network settings, look for the list of devices that had previous connected (and been assigned an IP address), and remove the Xbox from the list. Within minutes, my Xbox was back on the Internet, and downloading updates.