How to fix a ventilation pipe to the wall?
There's a number of things you could do, this is one I would do in your situation. These fans are made to fit inside sheet metal ducts. Go ahead and get 2 lengths of this stuff. You will end up with a joint somewhere in the room, but then the duct can run fully through both walls and there's enough slop in the fit that precise cutting is not needed.

Because of the joint, you will need a hanger support that anchors into the ceiling, which I assume is concrete. You would need a hammer drill to make the hole necessary to insert an anchor stud to which the hanger can be attached.

This is not the most attractive thing in the world, but it can be painted so that it's not too objectionable. Painting exposed ductwork is done intentionally sometimes as a desirable (to some) "look". You could also construct a soffit box to contain the duct, you end up with a sort of stair step on the ceiling. Given the extra work for this, it's easier to like the painted duct look.

To fill in the holes, get some cement backer board and cut to the size of each opening. Also cut holes to fit the duct or fan. For the size involved, the boards can just be glued in place with construction adhesive. You can also glue in some backup cleats behind the boards to increase the glued surface area if you like. The boards should be recessed about 15mm from the wall surface. The remaining space is filled with portland cement plaster in 2 or 3 coats. It is an obvious patch, but it sort of matches the wall and would be hardly noticeable if the walls are to be painted.