AFTV2 - Custom DNS Server setting does not work on WiFi connection

I have noticed on the AFTV2 that custom DNS setting only works on the WIRED network. If you try the same on the WiFI connection (by clicking Advanced in the popup window) ... it will pretend to accept the custom DNS ... but instead rolls back to the DNS supplied by the DHCP server.

I need the custom DNS to work on the WiFI connection. My Apple TV box also has a custom DNS - so Amazon need to fix the bug .. without me having to root the device etc.