Can’t Check In Online for China Eastern — Only Have Ticket Number Starting With 297

I’m trying to check in online for a China Eastern flight, but the website keeps asking for a ticket number starting with 781. The problem is: my ticket (booked through Opodo) only has a number starting with 297 — which belongs to China Airlines.

It looks like my ticket was issued by one airline but the flight is operated by another, and now the check-in system won’t accept it. Checking in with my booking reference also doesn’t work, and the France contact number seems disconnected.

Has anyone had this issue? Is it normal that online check-in doesn’t work with an interline ticket? Should I just check in at the airport with my passport?

Thanks for any help!