Ipad air 2 not connecting to 2.4 ghz but connecting only to 5ghz wifi

Hello,

Try resetting iPad's network settings in Settings > General > Reset > Reset Network Settings. Be certain you do not select Erase All Content and Settings as this will erase all of your information. It is a good idea to perform a backup of your iPad prior to resetting its settings.

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network - Apple Support

A restart of your WiFi Router may help - as, sometimes, they can inexplicably block some devices from connecting - or one of the radios can malfunction.

The best way to restart the router is to simply disconnect power from the router for at 30 seconds, then reconnect power to restart. After the router finishes its restart (which may take several minutes), force restart your iPad:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.