I was having the same issue with ios 11 (play music through the amp, but not connected so I couldn't control any settings). And 11.0.1 wasn't helping either. Wasn't sure if the Line 6 app wasn't working with ios 11, or ios 11 broke something. The app and my Amplifi 150 are both up to the latest versions. Still no luck.
My android phone worked fine with my amp.
I just upgraded to ios 11.1 beta and the app and amp now connect and everything works. Yippie.
Not sure what fixed it. I had tried rebooting my phone (even a more aggressive reboot, power/home, power/volume down on the 7) and that didn't help. I didn't try uninstalling the amplifi app and re-installing the app. Some of you with this issue might want to try that before upgrading to a new ios version, especially a beta (like I just did). Maybe it was whatever happened on the phone during the update. Maybe it's something new in ios11.1 that makes it work again. Not sure. But mine is working again and I'm happy.
But even if it is the ios 11/11.0.1 problem and 11.1 fixes it without a new version of the Amplifi app, that means soon, when Apple releases 11.1 to the masses this might fix things.
I'm not recommending people load beta ios versions on their "daily driver" phones, but if you google "apple beta software program" you will find it and it's free. Not recommending it, it's not for everyone. But hopefully things will sort themselves out naturally when 11.1 official.