How can I select 5GHz ac wi-fi when it's signal strength is less than b/g/n?

Open AirPort Utility on your Mac (Finder > Applications > Utilities > AirPort Utility)

Click on the AirPort router

Click Edit in the smaller window that appears

Click the Wireless tab at the top of the next AirPort Utility window

Click Wireless Options at the bottom of the window

Enter a check mark in the box next to 5 GHz name

This will add "5 GHz" to the name of the wireless network by default

Click Save

Click Update to save the new settings and allow a minute for the AirPort to restart

Now the 2.4 GHz network will be the name that you were using before and the 5 GHz network will be identified with the same name with "5 GHz" added. To connect to the 5 GHz network, you will need to select it from the WiFi menu at top of the screen on a Mac. If you are asked for a password, it is the same password as the 2.4 GHz network.

I find it much easier to use Apple's default setup of having both networks use the same name and allowing the wireless device to choose the best signal with which to connect. If my Mac is close the AirPort, it connects at 5 GHz. If I move the Mac laptop a few rooms away, it will automatically connect to the stronger and faster 2.4 GHz signal at that location.

But, you may disagree.....and prefer to have two separate networks and manually switch between them. Some users swear by using a separate 5 GHz name.....and some users just swear when they try this, because it makes things more complicated for them.

Connecting to 5 GHz does not necessarily mean that you will have a better or faster connection than 2.4 GHz. You might......and you might not. For example, if you try to connect to 5 GHz from your Mac when it is a few rooms away from the AirPort router, there's a good chance that the 2.4 GHz signal is both stronger and faster at that location.

As I said before, 5 GHz works best when the Mac is in close proximity to the AirPort router.

Try using the separate 5 GHz name to see how it works for you.....and let us know how things are going.

Assign a different name to the 5 GHz network, then "point" your Mac at the 5 GHz network.

But, there's a reason why the Mac is not connecting to 5 GHz at the present time......your Mac thinks that the 2.4 GHz signal is stronger (and probably faster). Locate your Mac close to the router and restart the Mac to see if it pick up 5 GHz.

5 GHz works best when your Mac is in the same room as the wireless router, or you have line-of-sight with the router, so setting up the 5 GHz network with a different name is not going to work very well if your Mac is a few rooms away from the router.

The 5 GHz signal does not actually have to be stronger than the 2.4 GHz signal for the Mac to connect, but it must meet whatever criteria the Apple engineers have programmed in to the Mac operating system.

When 2.4 GHz and 5 GHz are about the same strength, the Mac will usually connect to 5 GHz when the Time Capsule is using the default settings. But, it sounds like the separate 5 GHz network option is working OK for you now.

If the 2.4 GHz band is causing interference, there is an option to turn off the 2.4 GHz broadcast on the Time Capsule, so it will only transmit a 5 GHz signal. You would only want to do this if all your wireless devices can connect to 5 GHz, of course.

If you are interested in turning off the 2.4 GHz signal.....and you cannot find the (somewhat hidden) setting to do so.....post back for more clues.