Not sure if it will work with all, but works in the public metro wifi in DC:
Browse to ipv4.jolla.com in sfos browser, once you reach unclickable ‘terms and conditions’ copy the URL and open in angelfish browser (android firefox doesn’t seem to work), this worked also in some random hotel wifi that I don’t remember the name of, if you have other tips pls share
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Have you considered Captive Portal Helper?
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I usually open neverssl.com
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http://example.com ftw here 
(maintained by the IETF)
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