The real solution is for the captive portal is to signal that there is a captive portal.
Unfortunately that has turned into an arms race: some operating systems switch to a different browser when they detect a captive portal. So captive portals try to avoid detection, etc.
Basically what you want is package that tries to detect a captive portal and when it detects one alerts the user and offer to start a browser that uses the DHCP-supplied DNS resolvers to interact with the portal.
Trying to access any IP directly also typically works, e.g. 8.8.8.8. The redirect to that IP following the login is then of course broken/timing out, but that’s not really a problem.
Such captive portals are a pain anyway, even more reasons for providers to stop doing this — hijacking DNS requests.
What's the way to circumvent this problem? I'm frequently on public wifi's, so I need to access AP login pages without issue.