Raskal1130 are you still getting the phone is not registered message when you check? If the update is not available in your area it should say.no updates available. Go to an ATT store and ask for a new sim card.
Thank you Scott Jacobs1. Everyone having this issue, I did what he had said. I have spoken to at&t multiple times. First they had me factory reset my phone, and the backup they told me to do did not work, btw, but luckily, I did not have many things on the phone as of yet. Please make sure your things are backed up. Anyway, that did not work, they tried re-registering the phone to the network, that didn't work either. After talking to many reps, they did not know how to fix the issue.
I ended up going to an at&t store, like Scott Jacobs1 suggested. Even after they gave me a new sim card and registering it to at&t, the phone was saying the same thing. I had noticed that under the status, it was not showing my phone number, as it was saying "unknown number". They again were suggesting I do a factory reset. For fear of losing everything again, I did not do that. Instead, I did something else. All it needed was a reset. I turned off the phone, waited 30 seconds to a minute, and turned it back on. It did acknowledge my phone number after that.
I then waited until the next day, as I had to wait 24 hours until I can check for an update. I was able to get an update, which wasn't Kit Kat. I tried again the next day, and I got it. Now when I check for one, it says there are no updates available. I don't get the not yet registered message anymore.
So go out there and ask at&t for a new SIM card. Tell them you spoke to customer service and you need a new SIM card for your phone. Make sure it is registered in their systems, wait a bit and restart your phone until it recognizes your phone number and try the update when it lets you. That is how I was able to get mine to work. I hope this helps everyone with this issue.