Selecting correct key

How do you choose which key to play backing track in. I play Eb instrument and have set the default on the app to Eb. I'm trying to play along to sheet music in key of G. Default key on ireal backing track (which I think is B - 5 sharps) doesn't sound right, neither does it if I change the key signature to G.

You have to set up the Global transposition to your instrument (Eb).
Go to song view, tap on the treble clef in the player controls at the bottom, tap the round button on the top left corner of the blue area and select Eb.