I got an Airport Time Capsule nearly a year ago. It supports the 802.11AC WiFi protocol, but until the iPhone 6 I didn’t have any devices that could take advantage of it.
I’d assumed my devices, even the majority of which are lowly 802.11N capable, would take advantage of 5GHz on the Airport Time Capsule by default. At least for me that definitely was not the case. Not even with the shiny new iPhone 6.
Turns out that’s quite easy to solve: Just access your Time Capsule through the AirPort Utility Click on the Wireless and then the “Wireless Options” button where you can add a 5GHz specific SSID that will use the same password as your primary SSID. Check the box next to 5GHz network name and add the SSID you want to use (if you don’t want to use the default name it creates).
Then click save and then click update. After your Time Capsule reboots, connect to WiFi on your devices using the 5GHz SSID instead of the primary SSID.
While the 5GHz connection is mandatory to get an AC connection, this isn’t an AC-specific recommendation. I didn’t have many issues w/WiFi before making this change but I had a few dead zones for streaming video (even if the WiFi signal reported as excellent, video dropped out quickly). At least in my case after making this change, I no longer had anyvideo streaming dead zones, even on the devices that aren’t AC-capable.