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Try connecting the Nikon to the iPad instead of the iPad to the Nikon. Or, connect in the reverse 'direction'.
Have you entered the Nikon camera's proper WiFi 'passphrase'?
(Or are you using Bluetooth by chance, on the iPad?)
(Speaking of which, are you turning Off the main carrier or data service or 2G/3G/4G on the iPad? that might be necessary, if you are connecting with WiFi.)