Sounds like you've configured a whitelist to only allow the Wii U to connect to the modem. I'm guessing this SURFboard is a model that has both modem and router functionality. You'll likely need to reset the router to factory settings. How you do that depends on the particular model. If it is a Modem/Router combo it might have a recessed factory reset button on it, likely requiring a paperclip or something similar to actually press.
Another possibility could be to use the Wii U Web Browser and browse to the router's IP address, however I doubt the connection issues you initially experienced were related to the settings you changed so wouldn't expect it to be able to connect.