If your device is slow to reconnect to the wifi after it wakes from sleep, disable APSD Mode found under Advanced>Wireless and set WMM to auto. For me, both settings were required in order for the devices to reestablish wifi connections quickly... observed on 4 different Apple devices: 2014 macbook pro, ipad air 2, iphone 5s and 6s. Just wanted to share this tip with the group.
Hardware: Asus RT-N66U
Firmware: tomato-K26USB-NVRAM64K-1.28.0510.2MIPSR2Toastman-RT-N-Ext
Hardware: Asus RT-N66U
Firmware: tomato-K26USB-NVRAM64K-1.28.0510.2MIPSR2Toastman-RT-N-Ext