As described, you appear to be having difficulty accessing “captive” WiFi networks.
Begin with basic WiFi troubleshooting:
If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network - Apple Support
WiFi and Internet connection difficulties are usually an indication that your device does not have a routable IP Address - your IP Address usually being assigned by DHCP. When this occurs, your iPad may only have a self-assigned link-local IP address that begins 169.254.x.x.
If you have a VPN App or profiles installed, or a security App such as Norton, these may interfere with DHCP - and prevent your device from obtaining a valid IP Address for the captive WiFi network to which you are attempting to connect. Without a routable IP Address, your Pad will not be able to retrieve the captive network logon page.
The resolution is usually simple; if installed, you may need to temporarily disable the Security App and/or VPN. After you successfully authenticate to the captive network, you can re-enable your VPN and/or security App as required.