Can't Disable 5G WiFi on AirPort Extreme?

As I said, you are not looking in the correct place. Unchecking the 5 GHz box simply assigns the same name to both 2.4 GHz and 5 GHz bands. It does not turn off 5 GHz.

Checking the 5 GHz box allows you to assign a different name to the 5 GHz band.

You want to turn off 5 GHz

Open AirPort Utility / Edit / Wireless / Wireless Options

Click on the 5 GHz Channel selection box and select Off

Click Save, then click Update

IF......you have the current "tower" shaped AirPort Extreme.....AND....your Mac is running AirPort Utility Sierra version 6.3.7......THEN.....you can indeed "turn off" 5 GHz and/or 2.4 GHz as well.

Or, stated another way.....if your Airport Extreme and Mac meet the requirements listed above....you are not looking in the right place in AirPort Utility to disable the 5 GHz signal.

Post back to confirm what version of AirPort Extreme and AirPort Utility that you have.

Yes, we understand you have an 802.11ac AirPort Extreme base station. Have you tried either of our suggestions on where to enable/disable the 5 GHz radio? Bob even included an image from the AirPort Utility to make it easy.

Please follow the step by step instructions that I posted above.

The bottom line here is that you want to turn "off" the 5 GHz channel. So, to do that, you would choose the 5 GHz Channel selection box and choose Off.

Which exact model of the AirPort Extreme do you have? Will you be using a Mac, PC, or iOS device to manage it?

To disable the 5 GHz radio, select the 5GHz Channel option, and then, select Off.