For the lady using a wireless DSL modem with the Airport Timecapsule extending, perhaps you can disable the wifi features of the DSL router and use the Airports to create the network to solve your problem?
I think Apple is trying to fix this but may be reluctant to admit anything before it fully understands the problem.
After several days, I am running successfully using the Airport Time Capsule to provide a multi band network where the Air connects on 802.11AC and my other devices connect on the band of their choice. The Apple TV 2 is connecting and functioning REALLY well. I'm guessing it's 802.11n now, kind of hard to tell. Everything is doing fine. The default network seems to be WPA2 on the Airport. My connected devices (right now) include the 2013 MBA, the old 2010 MBA, an old Roku box (802.11g I think), an Apple TV 2, 2 PS3s, an iPhone 4s, and a iPad 3. Everything is connecting reliably. Better signal strength too.
My AT&T dongle also had to be switched out, but I think that was just that the old one was failing after 2-3 years in service. I switched it out yesterday and so far so good and now I have LTE capabiity. The new one is so new that Sierra Wireless has no data on its website yet (which was problematic for the purposes of installation) and the "automatic" download of the comm manager did NOT go as planned. We had to go to the website (at the AT&T store) and manually download and install. It's working now. It's larger than the old one, my old one was a shockwave and the new one is called the beam. It has a tiny little digital screen on the dongle but it's slower to connect than than the old one. It actually shows how much data you have left on the tiny screen.
I guess what I'm getting at is that it's nice when everything goes well in the upgrade, but it never does. There are always issues unless you're an expert and sometimes even then.