Ive recently gotten into modelling with blender. ive created my first model. everything looked fine until i imported it into CS. the ingame light is shining through some of the faces onto the newly added cubes and planes. when i go to the render viewport everything looks good and the model is perfectly shaded and creating shadows onto the faces which are covered from the sunlight. but not ingame. ive googled myself to death and i still havent fixed anything or gotten anywhre near a fix. i thought i need to get the engine to produce shadows in the textured viewport but it didnt make ANY difference at all since im never exporting the light creating the shadows with the object.

here i have some screenshots of the issue in blender

blender texture viewport:
https://gyazo.com/6a7ffb343ecf89b9906b7b4b7b74fa3e
https://gyazo.com/77a4bec76a41c7bce236eb547a3b5299

when i export and import it ingame it looks exactly like in textured viewport.
some faces let the light pass through them.

blender rendered viewport: (how i want it to look like)
https://gyazo.com/37e64b4135ccefbf929367efbe82eec3

i hope you guys could help me out. i think its just a little thing i forgot to add but i just dont know what it is and at this point im so frustrated that i created my first discussion in steam.
thanks

EDIT: it seems like it cant be fixed :/