I tried the first and sixth solutions here and they didn't work; most of the others aren't relevant to me because I don't have a router.
I tried connecting my old computer to the network and it seemed to work--at least, I was able to get as far as the wifi sign-in page, which I couldn't with the new one.
And today I took the new computer to Starbucks, where lo and behold, it connects just fine, not only to Starbucks wifi but to the network (xfinity) that I couldn't connect to at home.
So the problem is not my computer, right? But probably the local router I'm connecting through, which I have no control over because it's xfinity's router and not mine. At least this is what I'm assuming. But I don't want to keep having no connectivity from home, especially because since getting the new computer I can actually stream movies and music now.
So I guess I should contact xfinity, although the last time I had a problem they were incredibly unhelpful. But could the problem somehow be on my end? I'm wondering how my old computer could still connect.
ETA: Some research has shown that I can get a proper internet connection for less than I'm now paying for hotspot access, so I guess it doesn't really matter.
Crossposted at Dreamwidth ( comments); you can comment here or there.