From looking at the manual for the V90, the roof-mounted dog guard works in a similar way to the XC60, which I bought and fitted myself.
I posted the fitting instructions here. Hopefully the V90 will be very similar.
https://www.volvoforums.org.uk/showthread.php?t=314852
And yes, it says what to do with those plastic hoses. Very easy to do, took me all of 5 or 10 minutes.