Mostly the same issue as you.
I have a business class ticket with a mixed booking on Iberia/Latam. For both transatlantic legs, I can't reserve the seat. When using the website, they show me the economy cabin on the inbound flight, and for the outbound flight, I attempt to pay for seat reservation but encounter an error when trying to do so.
On the mobile app, no reservation is possible for the inbound flight ("not available"), and while seat selection is available for the outbound flight at 0.-, I receive an error message when attempting to confirm.
I'm not sure if it makes sense for me to try calling.
Additionally, do you know how I can add my Qatar Privilege Club membership to the reservation so I can earn Avios? I wasn't able to add it during the booking.