Why cant i sync apple tv with my router?

I just installed a new wifi system with a cox supplied motorola modem and an ASUS RT-AC 68U 1900 Dual Band router. I have been trying to set up apple tv on my flatscreen. When I enter the wifi password to sync apple tv to the network it keeps telling me that the password is wrong. But I know this is not the case. I have synced 3 Macbook pros and 3 i-phones to this new wifi network without a problem. I have tried resetting the Apple TV box and also rebooting the router and the flatscreen to no avail. I think its the router thats causing this as I have purchased a new A- TV box and same problem.