The Definitive Guide that is stickied has a
tutorial
https://gbatemp.net/threads/the-definitive-guide-to-wii-u-hacking.396828/
Use TubeHax DNS!! This will still allow you to use internet if you want, and only blocks Nintendo's update servers. It is ran by smea, who is very reputable in the 3DS hacking scene.
Set up TubeHax DNS on your Wii U:
1. Go to Wii U Menu/System Settings/Internet/Connect to the Internet
2. Input your router's SSID and password (typically WPA2-PSK AES)
3. Input settings as follows:
IP Address: Auto-obtain
DNS:
Don't Auto-obtain
Primary DNS: 107.211.140.065 (TubeHax DNS)
Note: This will leave you unable to access YouTube
Secondary DNS: 0.0.0.0
Note: This should make your Wii U fail to connect to the internet if TubeHax fails, though it's not confirmed.