Nighthawk Router wifi will not connect to internet when cat6 are in the ports

I have comcast internet. Surfboard modem, nighthawk router. When the modem is connected to the router only wifi works corrcetly. If I try to connect and ethernet cord to any of the 4 ports on the back of the router, the wifi stops working. All the wifi devices remain connected (at least the iphones and ipads show they are still connected) but can't access any websites, thus not connected to the internet. Any suggestions on how to remedy this issue?