This exact problem happened to me.
Here's what to do:
-Set your router channel to channel 1, this has worked the best for me and still works for me. Go to Nintendo.com/support if you need directions on how to get to your router's settings
-Keep your router away from metal objects, and preferably high up, like on a shelf, this allows the signals from the router to travel more easily.
-2.4 Ghertz phone + wireless router = wireless internet doom. If you have these phones, get them as far away from the router as possible, better yet, get 5.8 Ghertz phones if you can.
-Sometimes you need to allow a new device to use your router with permission, this can easily be done by going to the router's settings.
Follow these guidelines and soon you'll be on your way to Wi-fi hapiness!*
*Results not guarenteed, well at least I tried!