Why is the Internet suddenly not working on my phone?

I have a Samsung Galaxy A5 (2017) and it's worked very well for the year and a half that I've had it. This evening, it won't access the internet any more. I'm baffled.
I've probably messed it up myself. I was fiddling with various settings to try to troubleshoot a problem earlier today and may have turned something off that needs to be on or vice versa but I'm at a loss to figure out what.
When I try to reach websites I've never visited with my browsers, I get told I'm "not online" or...

Why is the Internet suddenly not working on my phone?

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