Before you ask:
My modem is not old - I think I got it last year.
I've cleared my caches etc in both Firefox and Safari.
Network diagnostic shows that everything is working.
Signal shows as strong, but nothing happens.
I want to try and avoid the pain and misery of Technical Support from ISP or Modem makers, so any advice or thoughts you might have would be welcome.
Oh, and another strange thing - if I go into System Preferences "Network" (I'm on a Mac), I get a message "settings have been changed by another application" which then refuses to close and the only way I can get rid of it is to shut down the computer. I don't know if these phenomena are linked.
Another question for Mac users - is there any way to clear settings and go back to some time in the past? Windows had that - I don't remember what it was called, but it was a good way to recover from some unwise download or something.