To turn on 24-band WiFi on an Apple device, first make sure the device supports the 2.4GHz band. Then go to the device's Settings menu and find the wireless LAN options. Here, you have the option to connect to one of the available 2.4GHz WiFi networks.
If you don't find a 2.4GHz network, make sure your router has the 2.4GHz band enabled and no limits are set. If the problem persists, try restarting the device and router, or contact your network service provider for further assistance.