You need to break-apart the wings from the joiners and then drop the wings back on the bed.
All things considered, this is a pretty poor quality model, especially considering who it comes from (who should know better).
For a plane that is made out of flat surfaces and triangles - the files are absolutely massive... 68,000 elements for a wing, 90,000 elements for the front piece, and a whopping 72,000 elements just for one of the joiner pieces - and there are three of them! Perhaps this model was meant to be printed in 1:1 full scale?
There is also no way on earth that the pictures are of the model files that were published - there are absolutely no seams on them where the pieces join together.