A lot of those captive portals use the HTTP Redirect method, which fails when the HTTP URL it intercepts is redirected to the equivalent HTTPS URL. Which happens to be a large fraction of the web nowadays. I find I have to remember the URL for a site which doesn't use HTTPS to make those portals work.
On top of spaghetti all covered in cheese, there was this tiny ad:
how do I do my own kindle-like thing - without amazon