Ipad not connecting to hidden SSID wifi

being that the whole hidden ssid is a fools security and people scanning for wifi networks find them all the same not that many use the feature anymore

but have you attempted to set it not to hide ssid and connecting so the ios device knows and auto join the network and then afterwards hiding it once more?

And you said that you have reset the network settings on the iPad. I'm guessing that you have the correct security setting configured when typing this into the iPad? Have you tried restarting the router?

Are there any special characters in the SSID for the network? Is it named something simple? That is about my last thought regarding connecting to a hidden SSID. See if this helps? Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support

At this point my recommendation would be to make a Genius Bar appointment at the nearest Apple Store to have the device examined by Apple. They can also assist with any other troubleshooting for the connection.

When you attempt to connect the iPad to the hidden SSID network, do you have any issues entering the correct SSID into the Wi-Fi area? You say that it immediately disconnects, but do you receive an error?

What I meant about security was that when you select Other for a wi-fi network and have to type in the SSID, it asks you for the Security protocol and the password as well. When you un-hide the SSID, did you see what security protocol it was so you could match that selection when you type in the SSID?