HTTP status codes / 5xx — Server errors
A captive portal wants you to log in to the network.
Issued by network infrastructure (hotel/airport Wi-Fi, corporate guest networks), not by the site you tried to reach: authenticate to the network first. Defined so clients can detect captive portals.
Open a browser, complete the portal login, retry. In mobile apps, expect this when testing on guest Wi-Fi.
Server-side note: Open a browser, complete the portal login, retry. Expect this when testing on guest networks from a phone.
The fastest way to pin down a 511 is to reproduce the exact request and inspect what actually went over the wire — status, headers, timing and body, without your app code in the way. That is what an API client is for; ReqPad does it from your phone, with every request saved to history.
500 Internal Server Error · 501 Not Implemented · 502 Bad Gateway · 503 Service Unavailable · 504 Gateway Timeout · 505 HTTP Version Not Supported — or the full reference.
Build the request, send it, read status + headers + timing — on your iPhone. Free to start.