locked out of Gmail after switching to library ISP
 

I recently lost Internet access and borrowed a mobile hotspot from a public library. When go to Gmail using it and try to log in to an account, I receive an intercept saying "
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"Couldn't sign you in
Try using a different browser. If you’re already using a supported browser, you can refresh your screen and try again to sign in."
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Why should this occur under these circumstances, and what can I do about getting back into my Gmail accounts?

All advice appreciated.

Are you connecting to the internet via the library? Then I can imagine that because of public use, things have been blocked for security reasons.

No, the library lends out little wireless gadgets provided by T-mobile that can be taken home and will work anywhere to provide a sort of portable WiFi, presumably via their cell network. I'm not talking about being locked out of some specific and possibly dodgy site. I'm talking about not being allowed to log in to Gmail (or most of my other email providers) -- that is quite a different thing.

Try using an email app instead of web site.