Every once in a while, my iPhone 6 running iOS 8.2, refuses to automatically bring up a captive portal authentication page. Bringing up a browser didn’t help, because my phone would automatically route data over the cellular network.
The workaround is as simple as disabling "Auto-Login" for the specified network.
- Go to Settings → Wi-Fi → <Network> Info
- Disable "Auto-Login"
- Open Safari or any other web browser
- Go to http://captive.apple.com or other unencrypted page (e.g. http://www.msftncsi.com, http://nist.gov)
If you use LastPass, this also allows you to use the LastPass extension to auto fill captive portal authentication fields.