Hello Chris-MacBookAir!
Thanks for reaching out to the Apple Support Communities. I understand that you’ve been unable to log into a Wi-Fi which uses a captive portal login page. I know how important it can be to access networks which require entering in credentials or accepting terms of service.
If you’ve been using a DNS (Domain Name System) server which you’ve manually entered in System Preferences > Network, you’ll need to remove it. This has occurred with my own MacBook Air as well as with a friend’s when using public DNS servers. Follow the steps in this article to access the DNS settings: Specify a DNS server on Mac.
Highlight any DNS server which you’ve manually entered, and then click on the [—] to remove it. Once all are removed, the router for the network you’re connecting to will assign the DNS server, and will present the captive portal login page.
If you haven’t manually entered any public DNS servers, try restarting your MacBook Air into safe mode with these steps: Use safe mode to isolate issues with your Mac. Once at the desktop, please try connecting to the network, and see if the captive portal page appears. If it does and you’re able to login, try restarting your MacBook Air and see if you can connect again under the normal startup.
Let me know if this is helpful.
Cheers