I would try to manually download the IPSW for iOS 11 separately.
You can download from [http://www.iclarified.com/8500/where-to-download-ipad-firmware-files-from|here].
Once you have downloaded this file, launch iTunes and put the iPad in DFU mode again.
When you plug your iPad in as DFU, it will ask you to restore. Select "no".
You will then be shown a screen with a "Restore" button
• on a Mac, hold "Option" key down + Click the "Restore" button. Then select the manually downloaded file from above.
• on a Windows machien, hold "Shift" key down + Click the "Restore" button. Then select the manually downloaded file from above.
As a last resort, use [http://www.3u.com/|3uTools] to flash the firmware.