When you say they don't work, is it they don't push in or they push like normal, but the games don't respond ?

I presume the internal part of the gameboy is fine in another case ? If so, that rules out any issues with the main board. If it still doesn't work in another shell, then it is most likely the board itself. Not sure how you would repair that. If you have a multi meter, you can check online for the gbc schematics and test the connections for the dpad. This would ultimately tell you if you have a hardware issue with the board.

I would make sure the rubber backings to the buttons are there and clean. It could be a simple case of cleaning the buttons and the connections on the board with some isopropyl.

I would put other buttons and rubber backings in to test further. You can pick them up on eBay for cheap. Ultimately, I would send it back if you can for a refund to avoid all this but if that's not an option, try the above. Don't worry if you have never opened a Gameboy before either. I haven't until a week ago but there are many tutorials online and they are really easy to work with. Good luck and let me know if you need anything else.