@leocg by logging in using the prompt, does that mean clicking the "connect" button? If so it just loops back to trying to open the login page, and then showing the "Connect to Wifi" prompt.
I'll add that when I connect to the university wifi, my computer automatically opens the browser with an address that redirects it to another address which will then bring me to the login page. As I made Opera the default browser, it will Opera and then uses the first address, but the redirect to the 2nd address will be blocked by Opera's "Connect to Wifi" prompt.