Check if the name (SSID) and security key of the wireless access point are entered correctly.
Check if the access point and/or modem are switched on.
Check if there are other devices within range that are emitting wireless signals. These could cause interference.
Move closer to the access point and make sure there are no obstructions between the Wii U console and the access point.
Try changing the channel on your wireless access point. Please refer to the instruction booklet that came with the access point for information on how to do this.
Manually enter the IP address and DNS settings in Wii U connection settings