Nighthawk X4S R7800 unable to block access for one unknown device

I have been using this router for a short time and checked out the Attached Devices. I identified and labeled them all except one. It is listed as n/a other. It has and IP and MAC, but I don't know what it is. It is attached via wifi 5g. I went to the access control page and thought I would block it and then find out what it was when something didnt work. Access Control is checked on. I checked the box next to the device I want to block and selected Block. A popup asks if I am sure and I select OK. It refreshes, but when it is done it is still Allowed. I was able to block and allow other devices, but not this one. I tried the Nighthawk app too and when I try to Pause the device it says "please try again later" It does not say that for other devices.

Anyone see this issue or know why I can't block certain devices? 

I am using the latest firmware 1.0.2.68