This is starting to become a problem, because more and more websites are only accessible through HTTPS (TLS). Modern browsers remember whether a website supports HTTPS and will use the HTTPS version, regardless of what you enter in the URI bar. Techniques such as HSTS ensure that browsers will always use the encrypted version of a website. Newer versions of the HTTP protocol such as HTTP/2 and HTTP/3 don't strictly require encryption, but all major browser vendors have decided to only implement them for HTTPS connections.
If you try to visit Facebook or SuperUser, for example, your browser will automatically use an encrypted, authenticated connection, and when the captive portal tries to redirect the browser to the login page, the browser will detect this manipulation and throw an error. Normally, this is exactly what you want, but in this case, it will prevent you from logging into the captive portal and thus from using the Internet.
In this case, you can solve the problem by visiting a website that you know is "insecure", i.e. doesn't use HSTS, SSL/TLS, or HTTP/2.0 (which requires TLSthe standard specifies both HTTP and HTTPS, but browser vendors have decided that they will support only HTTPS for HTTP/2 going forward). The above-mentioned URIs should do the trick, but there is actually a website which some nice people have put up that serves exactly this purpose, and whose URI is easy to memorize: http://neverssl.com/.
NeverSSL does exactly what its name suggests: it is simply a completely useless website whose sole purpose is to never use SSL/TLS, HSTS, HTTP/2.0, QUIC or anything other than un-encrypted, un-authenticated, insecure, plain HTTP/1.1, so that the captive portal can intercept the request and redirect to its login page.