I have booked a flight on Qatar website. However the first leg to Doha is on a BA codeshare flight so I could only book seats on the other 3 QR segments and although the booking appears on my BA app too it says seats assigned at check-in so not even an option to pay even though seat selection was free on other legs.

Never flown with mixed flights before so just wondered when it comes to checking in for the flight, do I check in on-line with QR or BA app/website or which airline check-in counter do I go to?

Many thanks in advance.