Suddenly began having this problem after upgrading to Xcode 8.3 and iOS 10.3:
When my iPhone 7 is connected to my Macbook Pro (via standard lightning cable), my phone loses wifi. It shows "T-Mobile LTE" but is unable to access the internet. As soon as I disconnect it from the lightning cable wifi turns back on. I'm unable to debug my app (which requires Internet) so I'm stuck. Have tried everything I could think of, turning off Wifi, resetting network settings, restarting laptop and phone, etc.
As soon as I disconnect the phone from the laptop, wifi access turns back on and the app can access the Internet. But since the phone is no longer connected to my laptop, I can't debug the app.
I also tested on an iPhone 6s -- same thing happens.