The CaptivePortal is test, used by Apple device, when joining a WiFi network. The test tries to determine whether or not you have joined a “captive network”, that will require additional actions to sign-in before access to the wider network (or internet) is granted.
For example, many hotels and public transport hubs have WiFi networks that can be used by visitors to these locations. Some (if not all) will require that you actively accept Terms Of Use before being granted access. Others may require you to “register” and/or pay a fee before being able to continue.
If you have a VPN or Security App (e.g., Norton or McAfee) installed, these may be interfering with the Captive network login. If installed, these Apps may need to be disabled or removed from your iPad.