Using GB iPhone as a sound module

Hi. New to GB ( old time Logic user). I want to use GB iPhone as a song module., live on gig.

I have 3 tracks of a song recorded, want to use just the bass tone, played from midi kybd., live.

My set up is keyboard midi to usb hub, hub to camera connector ( Apple) , to iPhone. All works great recording. And I can hear song playback on earphones connected to the hub.

But I cannot hear a track sound ( enabled) played from midi kybd. ( no song playing ) thru the headphones. Seems this set up would allow…..am I missing something simple?

Thx for any pointers.

Bud