Hi I have an iPhone 5S and have upgraded via AT&T to the iPhone 6. Sim card came already in the device. Turned it on and synced with iTunes, all good. Problem is, the iPhone 6 has "No Service" for cellular and my old iPhone 5S has retained the service. Do I need to activate the iPhone 6? Nothing came with the phone to suggest I needed to do anything. I tried https://www.wireless.att.com/activations/ but get a "Gateway Timeout" every time... Any ideas?