Pi 3 Unable to login to captive portal

[Supay]

Good morning,

I have a Pi 3 with ArchArm installed using the armv7h build. It works very well, and I have kept it as clean and minimal as possible. I can connect to wifi perfectly happily, except at my workplace where they use a captive portal which requires a username and password to login with. It is one of those third party provided systems that goes straight out on its own connection, rather than being part of the internal network here. They are based at http://hub-connect.net/

I am using wpa_supplicant, dhcpcd and resolvd, and I am able to request and be issued a valid IP lease from the wifi hub, with a timeout of 600 minutes. I can see the gateway and the local DNS, and can query the provided DNS for details on external addresses. I have ensured that my local time is synced and up to date as well.

Any queries to web addresses either from the command line or in a browser, result in a redirect to a captive portal login. I have a valid login and try using this, but every single time it pauses for around 10 seconds then reports "Request timed out, check your connection to the Hotspot". Checking my current connection, it still shows active and I can query the DNS as above.

I have tried numerous browsers, with the same result. I have tried setting overrides to have those browsers report as running on Windows or to be IE, with the same result. I have cleared all browser data, with the same result. I've tried a bunch of manual configurations to the point where I actually got in a muddle and had to wipe the device and start again, all with the same result. Colleague's on Windows laptops and iPhones can connect with no issue. My Android phone using Firefox can connect with no issue. However, I also tried Raspbian and that too could not connect. And my colleague's Ubuntu 18.10 laptop also cannot connect.

Does anyone have any advice or similar experience please? Is it possible that, based on three different distros running on two types of hardware, they may block Linux entirely? Or do they have some really shitty setup that is configured in a way that means I have to do something very specific to connect when using Linux? Any advice would be very appreciated.