I had this same problem on a Nintendo 3DS XL, reseating the wifi card did not fix issue. Bought a replacement card and that did not fix it either. Come to find out it’s just an issue with putting enough pressure down on the card. So I added an extra bit of padding on top of the wifi card. Pushed it together and the 3DS recognized the wifi card again and started working perfectly fine. So for all the people that tried reseating the card and that not working try adding extra padding on top of the card so it pushes it down more.