Thank you! Any help on what to actually say to Qantas? I keep trying hoping to get a rep. who can help but so far no luck...

Hi guys,Long time lurker, I'm flying Qantas SYD NRT bus with Amex via Cathay via Asia Miles. I'm trying to add my son ( < 24 months lap infant ) to the booking.Qantas is saying as the ticket was initiated with Cathay they can't add him to the booking.Cathay is saying as the flight is with Qant...