Can't get my MacBook Pro to the "Agree" page on a public wifi hotspot
Try manually accessing a page using http, not https, and try it with multiple browsers.

The captive portal test just tries to pull a file from one of Apple's servers. If it doesn't match what it's expecting, it knows it's not on the open internet and shows the login page.

Some providers let the test through to avoid this behaviour. If you open up a web browser and try to access a page via http (not https) you should see the login page.