Assuming you're on iOS 9 or later, on your iPad, make sure 'Wi-Fi Assist' is turned on. It's in Settings > Mobile Data (Settings > Cellular in some regions) section.Basically, it allows your device to continue using your mobile connection for data even when WiFi is present if the device determines that the WiFi is not offering a working internet connection.
This setting does default to 'on' though... so you're either still on an iOS lower than 9 (why?) or you've turned it off!