Time Capsule doesn't have AC radio option

Looks like you are trying to use an older 3+ year old PC version of AirPort Utility.....which is not officially supported......with the new Time Capsule.

You might want to review the System Requirements for the Time Capsule from Apple's specifications:

System Requirements

Setup and Administration

iPhone, iPad, or iPod touch with iOS 6 or later and AirPort Utility 1.3

or

Mac with OS X Lion v10.7.5 or later and AirPort Utility 6.3 or higher

http://www.apple.com/airport-time-capsule/specs/

Apple locked down the settings to Automatic.....so there is no Radio Mode setting at all on the newer versions of AirPort Utility for the Mac or iPhone/iPad.

"ac" is always enabled on the 5 GHz band, as is "n", "g" and "b" for the 2.4 GHz band. No way to change that.

Apple has not updated the Windows version of AirPort Utility for almost 4 years. If you have AirPort Utility for Windows 5.6.1, that is the latest version.

If you have a Mac that is "ac" capable, you can use that to verify that it is receiving "ac" signals. Any non "ac" capable devices will connect to the normal "n", "g" or "b" bands.

Yes, if all of your devices are capable of receiving "ac" signals.....and they are in close proximity to the Time Capsule. Line-of-sight is best.

"ac" is a two way street......both the router and the receiving device must be capable of handling "ac" signals. If you have a Mac, it is easy to check to see if it is receiving "ac" signals. Simply hold down the option key and click the WiFi icon up at the top of the screen. Look for the PHY mode setting.

Most the devices on my network are not capable of handling ac signals

Wireless devices will connect to the best signal quality automatically based on their location in relation to the Time Capsule and their own capabilities.

"ac" is carried on the 5 GHz band, which is not nearly as strong as the 2.4 GHz band, so for "ac" to work well.....with "ac" capable devices.....you will need to have line-of-sight or close to it to realize the benefits of the higher speed "ac" signal. If there are a few walls in the signal path between the Time Capsule and "ac" device, you probably won't get "ac" speeds......you will get "n" speeds.