Have you done the following (Related to a Vanilla Wii U.)
- Is your internet good enough to play on the Wii U
Yes = Continue
No = Do not, or get a better internet provider
- Have you turned the Wii off, and on multiple times? Has the issue worked the problem?
Yes = Your Wii internet connection to the server should work now
No = Continue
- Have you verified a server maitence is taking place? Is the maitence finished?
Yes = If it's finished you should be able to play just fine. If not try re booting your internet provider, router, etc...
No = Wait or proceed below
Have you refreshed your internet router?
Yes = Wait 20 seconds, and then plug the cable back in to reactivate the router. This should solve the problem
No = Do it. As at this point you can't really do anything else :/.
https://m.reddit.com/r/splatoon/comments/3bnxq9/splatoon_wont_connect_to_internet/