I Have the same problem with my iPhone 5s and my IPad Pro. It has never been able to auto connect on my iPad since I updated it to iOS 10 right as I got it, however my iPhone 5s will not auto connect after updating to iOS 10. What's odd is that my 5s was on all of the iOS 10 betas, and it only stopped working on the final release version. My Mac connects fine, and all of my friends with an iPhone 6 or above, or Android phone are having no problems. Even my 3DS and Wii U can connect without a problem! The most annoying part for me is that the password is a long string of letters, numbers, and symbols, so I have to copy and paste it from notes every time. And just like what everyone else is saying, always fails to connect on the first try, works fine on the second try. I have lots, and I mean HUNDREDS of connections I don't want to lose, so I don't want to reset my network settings, but most people have been saying that doesn't work anyways. This is a huge iOS 10 bug, and it better be fixed sometime soon.