London underground wifi

Ok, the answer to this problem is actually really simple. The "THREE_WIFI" network (used by both Three and Smarty customers) uses a security protocol called EAP-AKA. To be able to connect to this network, your phone must understand that security protocol. It turns out that OnePlus were really late to implement EAP-AKA into their stock system (i.e. the system that OnePlus puts onto their phones at their factory).

To give you an idea of how late they were, EAP-AKA is defined in a document called RFC 4187 which was published in January 2006. The system that I was using on my Oneplus 5T was released in 2017, and it still did not contain EAP-AKA.

In the end, I replaced the standard system on my phone (OxygenOS v10/Android 10) with a "third-party" system (LineageOS v18.1/Android 11), and I can now connect perfectly to THREE_WIFI.

Hope this helps, Eek.