The "ac" signal is on the 5 GHz band and the normal 802.11b/g/n signals are on the 2.4 GHz band.
5 GHz is capable of much higher speeds than 2.4 GHz, but the downside to 5 GHz is that the signal is much weaker than a 2.4 GHz signal. As a result, 5 GHz really only works well when the router and Mac are in the same room, or close proximity to each other.
Your Mac is always looking for the strongest signal with which to connect when it powers up or comes out of sleep.
By default, Apple assigns the same wireless network name to both the 2.4 GHz and 5 GHz bands. So, unless your Mac is very close to the AirPort router, it will likely connect to the 2.4 GHz band.
Do you have both the 2.4 GHz and 5 GHz bands set to use the same wireless network name? We ask because there is an option in AirPort Utility to assign a separate name to the 5 GHz network. Then, you can "point" your Mac at that specific network.
But......important.....you must also change the Connection Order on the Mac in System Preferences to make sure that the 5 GHz network is the first priority for the Mac with which to connect.
Post back if you need more help with assigning a separate name to the 5 GHz network and setting the Connection Order for networks.
None of this will guarantee that the Mac will connect to 5 GHz consistently if it is normally a room or two away from the router, but it will improve your chances.