Perhaps this support page will be helpful with troubleshooting:
Use captive Wi-Fi networks on your iPhone, iPad or iPod touch – Apple Support
Apple reportedly uses this URL to test for a Captive Network:
Success
Issues with connecting via a public WiFi network, that employ a captive network portal through which you must initially authenticate and/or agree to the Operators terms of service, are almost always due to either your device settings - or some other App installed on your device that intercepts your network traffic.
Starting with your iPad settings, Apple's Private Relay Service can sometimes interfere with Captive Portal Authentication. Disabling Private Relay can remove the issue - at the expense of your privacy:
Settings > [Your Name / AppleID] > iCloud > Private Relay > Private Relay - set to OFF
Next, if you have a VPN App or profiles installed, or a security/AV/firewall App such as Norton, these may be interfering with the network stack (e.g. DHCP and DNS address resolution). While these Apps are active, you may be unable to authenticate via the Captive Portal. If applicable, the resolution is usually simple; if installed, you may only need to temporarily disable the Security App and/or VPN.