Having the same issues as you guys with IOS 11.0.1 running latest Amplifi app and my Amplifi 75. Just noticed that IOS 11.0.2 is out so downloading now and keeping fingers crossed!
EDIT: After updating to IOS 11.0.2 the Amplifi app kept crashing but after uninstalling and re-installing it I now get a solid connection with my Amplifi 75 again :)