How to select seats on OneWorld airlines booked through Qantas?

Well, after months of stress I have now got my RTW trip ticketed! The problem now is that I can’t select business class seats on Iberia flights - not on the Qantas website or on Iberia (although I can locate the booking on Iberia). I had the same problem on Alaska Airlines but their website had a chat option and the agent was able to reserve the seats for me. I’m also travelling on British Airways but I can’t select seats at the moment unless I pay for them.
Can anyone suggest an option for booking the Iberia seats?

BA is known to charge for seat assignments even if you are in business. Iberia’s IT is dodgy at the best of times - could be free if you call to select or needs to be paid, so better check!