The iOS 7 betas have now expired, which means anyone still running one won't be able to use their iPhone, iPod touch, or iPad, and won't even be able to get past Activation if they try. That's the reality of betas, and why only developers should be using them. But, stuff happens, people get stuck, and even developers can forget they're running betas. So, if your iOS 7 beta expired - whether you know it or not - and if you're experiencing activation errors, what can you do? Easy, just update to the release version of iOS 7. And luckily, iMore told you how to do it weeks ago.
You can try just updating first, you can try downloading the firmware files on your own, you can try an automatic restore, and all else failing, you can try a DFU restore. Here are the details: