1) Go to your LAN PC and ping all wireless devices connected to your router. Ping should tell you if there's delay, loss of packets or devices cannot be reach.
2) Change router channel, there are instances that wireless phone (not cell phone) at 2.4mhz can kill WIFI signal,
3) Check if there are other Wifi signal with same channel being use.
4) If it just COD problem it can be server error, happens a lot or cheaters get banned in COD , just to rule out router problem, go to wii setting and check and verify connections, also go to your internet channel, weather channel or shopping channel it should be able to tell you that the connection is there.
Try to unplug the router for a few minutes then plug it back in and try connecting.
If that doesn't work reset your router to default settings. Then try connecting without any security set up (no encryption).
If that doesn't work then it is possible your wireless router is defective.
If it works then set up your security. I would use WPA not WEP if possible. I have had nothing but issues with WEP disconnecting my wireless stuff.
After you set security up I would setup the Wii MAC address in your router so it gives a certain IP to the WIi system only and doesn't assign it to something else that may cause an IP conflict.