asus rt-ac66u "could not connect to Apple TV"

hi guys,

i recently purchased a new router (ASUS RT-ac66u) and I can't get my 3rd gen Apple TV to work with it, even though it used to work perfectly with my previous no-name router issued by my ISP.

My MacBook can see the Apple TV on the network, as it shows up in the menu bar, but fails to connect to it bringing up a dialog box of a cinema display and saying "could not connect to Apple TV".

If I try connecting with my iPhone 5s it just drops the connection immediately, if I keep hitting the Apple TV as a target for Airplay from Control Center, it just makes the phone reboot.

I tried custom both older firmwares for the router and custom ones (merlin and tomato), but couldn't fix it. Currently everything is set to default in the router, and 5ghz mode is off so as to make sure all devices are on the exact same network. All my other devices are up-to-date too.

Any help would be greatly appreciated. Many thanks