Thank you for the idea about Facebook, but how you contacted them on Facebook? I just called - and it was long and Rep was not helpful at all; he even does not hear about DL Diamond benefits while flying LATAM.
Nope, it was regular economy, but their app wanted me to pay for any seat choice despite DL FF# was in the system.
I Asked them to reserve seats with extra space for me. they incorrectly told me it was not possible to reserve, and i would need to wait until checkin.
So i asked them for another set of seats, which i knew, based on seatguru, were just regular seats. They reserved and there was no further problem. The seat reservation stuck.