Q: WIFI is Grey and can't turn off FIND MY IPHONE

Howdy punyawee,

I understand that you're no longer able to turn your iPad Air's Wi-Fi on. In such a situation, if you haven't already, resetting the devices network settings can help restore Wi-Fi functionality.

Check if the issue still exists after each step:

1. Make sure you have the latest version of iOS. 
2. Tap Settings General > Reset > Reset Network Settings. This will reset all network settings, including Bluetooth pairing records, Wi-Fi passwords, VPN, and APN settings. 

If you still need help, contact Apple for support and service options.

If Wi-Fi is grayed out or dim on your iPhone, iPad, or iPod touch
https://support.apple.com/en-us/HT204324

Regards!