The redirection will result in a security warning from most modern browsers because the original HTTPS request has been redirected to either an insecure open portal or to an HTTPS portal that is using a different SSL cert than the original request.
If the user selects continue after the warning, the Guest Portal will come up so that they can sign into the network.
Websites that use HTTP Strict Transport Security(HSTS) will not allow HTTPS redirect.
A server implements an HSTS policy by supplying a header over an HTTPS connection (HSTS headers over HTTP are ignored).
For example, a server could send a header such that future requests to the domain for the next year (max-age is specified in seconds, 31536000 is approximately one year) use only HTTPS: Strict-Transport-Security: max-age=31536000; includeSubDomains;.
When a web application issues HSTS Policy to user agents, conformant user agents behave as follows:
Automatically turn any insecure links referencing the web application into secure links.
For instance, http://example.com/some/page/ will be modified to https://example.com/some/page/ before accessing the server.)
If the security of the connection cannot be ensured (e.g. the server's TLS certificate is not trusted), show an error message and do not allow the user to access the web application.
The HSTS Policy helps protect web application users against some passive (eavesdropping) and active network attacks.
A man-in-the-middle attacker has a greatly reduced ability to intercept requests and responses between a user and a web application server while the user's browser has HSTS Policy in effect for that web application.
More info: https://en.wikipedia.org/wiki/HTTP_Strict_Transport_Security