How can I refit the air tube?

My Ghone wall button / concealed access plate is connected to the "plunger" device in the cistern via a fine plastic air tube that should lift the plunger by a pneumatic action. Unfortunately the air tube has come off at the cistern end.

I can't see any way of reattaching the air tube with out dismantling the wall. There must be a way. Does anyone know?? The only access is via the front access plate.