The captive.apple.com error would normally suggest that you are trying to access the internet from a public WiFi hotspot; the URL is used by the iPad to detect internet connectivity. Errors relating to the address generally indicate a failure to authenticate the user via the hotspot operator’s paywall/terms page - prior to granting internet connectivity. As you are using a domestic Broadband service, the only other viable explanation is that your WiFi/Internet router is perhaps blocking access to some internet resources.
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:
https://www.apple.com/library/test/success.html
Are you selectively blocking any sites or protocols via your Router’s firewall? You’ll need to check the setup of your WiFi Router. Pay particular attention to any firewall logs - as these may provide useful clues.
A restart of your WiFi Router may also resolve the problem. To restart you home router, disconnect power - then wait at least 30 seconds before restarting. When you router has rebooted (which may take several minutes), force-restart your iPad:
- On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
- On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.
Changing the DNS provider settings on your WiFi Router - and/or your iPad - may also resolve the problem.
For recommended DNS servers (configured at your home router and/or your iPad), you might choose one of the following Recursive DNS Providers (IPv4 and IPv6 addresses included below). If your home network or broadband provider are “dual stack, supporting both IPv4 and IPv6 network connections (most now do), you should configure both:
Quad9
9.9.9.9
149.112.112.112
2620:fe::fe
2620:fe::9
OpenDNS
208.67.222.222
208.67.220.220
2620:0:ccc::2
2620:0:ccd::2
Cloudflare+APNIC
1.1.1.1
1.0.0.1
2606:4700:4700::1111
2606:4700:4700::1001
