Hello everyone I tested with my friend and solve this issue yesterday.
We tried some mobile devices including iPhone 6s, iPhone 5SE and iPad Pro etc...
It's only the iOS 9.3.5 can connect with Amplifi TT.
So, I update my iPad Pro from iOS 9.3.x to iOS 9.3.5 ( sorry I forgot what number is X)
And it is works.
Sorry we don't have android system can test.
hope it can help other people who has problem about Amplifi app shown device not connected when your bluetooth is already connected.
P.S. Sorry my poor English, hope you guys can understand what I mean.