How to get the protocol of MFi to be used BLE connection?

You cannot use EAAccessoryManager to connect to a BLE device. This is because EAAccessoryManager works with already connected BT Classic devices (connected as in paired and connected via iOS BlueTooth settings).

To communicate with a BLE module, you must use the CoreBluetooth API. Instead of a protocol string (which is used for BT Classic iOS connections), you must instead discover the GATT characteristic UUID's for your module, and use those to communicate with your device. These UUID's vary by manufacturer. To find them, either look into manufacturer documentation, or use an app such as LightBlue to find your BLE peripheral by it's friendly name, then test/discover the GATT characteristics from the app.

Make sure to read this thoroughly before starting: https://developer.apple.com/documentation/corebluetooth