The solution to the problem was unbelievably simple! To think of all the months I've spent is maddening. The problem became worse when I could not use the AppleTV device. So again, hours on the phone with the ISP techs and the Apple tech none of whom could/would delve into the problem. Apple said it was the ISP's problem and wouldn't help. The ISP said it was Apple's problem and couldn't/wouldn't help. All the port forwarding, which I learned on my own helped with the torrents, but not with the wireless network connection in my home. Today I called the ISP one last time to tell them "To hell with your cheap modem router--I'm getting my own." (Against the ISP's rules.) and I lucked out and got a tech who was competent, knew something, and more importantly, cared enough to help!
It took one radio button--ONE! Enable UPnP! The techs who installed the modem (when all the trouble started) had neglected to enable this feature, and I didn't know enough to check that out. (Why should I, when your ISP is the 'expert.'
It is fixed! I can now stream from my AppleTV! I can now cleanly download torrents and in turn upload, thus share!
Amazing! For months--not one ISP tech knew the answer or cared. The Apple tech said he could not give help with the router. I understood that!) But after searching on the Apple forums, I had discovered that others had had the same problem and the same error message. So Apple should have at least known the cause, even if they would not help me fix it. They could have said, "It's in your router. Enable UNpN." So there it is! Thanks to all of you who did try to help, especially Jose Brihuega, who spent lots of time on this.