how to enable both 2.4 & 5GHz on airport extreme Simultaneous dual-band

You can use your Mac to view ALL the WiFi access points, and the frequencies they are using

Option-Click on the menu bar WiFi icon

and select "Open Wireless Diagnostics..."

Select Wireless Diagnostics -> Window -> Scan

You will get a display of all the WiFi access points your Mac can see. It will tell you the frequencies, channels, RSSI (signal strength), Noise (interference strength), etc...

I would show you a screenshot, but I tired of blacking out my personal information 🙂 But it becomes very obvious once you see it.

You will be able to see if your Airport Extreme WiFi router is offering 2.4GHz as well as 5GHz.

Under the Wireless tab, click Wireless Options.

However, I don't think there is any method to enable or disable either frequency. You can establish a different name for the 5GHz network.

Is your Airport Extreme dual band?

Have you tried using Airport Utility/Wireless?

AirPort Utility 6.x: Set wireless options for your network