I went into my router settings and changed the SSID for the 5GHz band (the one that wasn't working).
It came up right away. Then I changed it back to the original name, because I like the name, and so that I wouldn't have to change all my other devices, none of which had any trouble on either band. It still worked.
Hope that works for others. For the record, I'd already tried safe boot and recovery mode without any joy.