If you are having issues after updating to iOS 10 on your iPhone 6, iPhone 6 Plus, or iPhone SE devices and you are T-Mobile users now you can fix this problem. Apple has released a fix that allows T-Mobile users to continue to connect to their carrier’s cell towers while also getting all of the iOS 10 feature, meaning now there is no problem for T-Mobile users using iPhone 6, iPhone 6 Plus, or iPhone SE devices.
How To Fix T-Mobile Connectivity Problem
If you’re a T-Mobile user in the United States and own iPhone 6, iPhone 6 Plus, or iPhone SE and you haven’t updated to iOS 10 yet, you can now head into Settings > General > Software Update to update to iOS 10. If you have already installed iOS 10 on your T-Mobile iPhone and lost cellular connectivity, head into Settings > General > About to grab the new carrier update which should fix the problem.
So in short just go to settings and update your iOS with the message coming in your device.