I'm sorry but the answer will have to be slightly technical as it involves routers and going into the router settings.
First try moving the router closer to the wii or vice-versa. If that doesn't work, try a few of the things in the guide on the link:
Most of the time going into your router settings helps and changing the wireless broadcasting channel to 1 or 11. Putting the MAC address from your wii internet settings into the MAC filtering settings on your router helps.
Hope this helps.