How to fix the WiFi on the Wii U

You need a computer with WiFi and ethernet, OS doesn't matter, as long as you have a web browser.

You need a portable wireless router, and a USB ethernet adapter that works with the Wii U.

TP-LINK TL-WR700N Wireless N150 Portable Router, Pocket Design, Router/AP/Client/Bridge/Repeater Modes,150Mpbs
Network Adapter, (Support Wii, Wii U; ASIX AX88772 chipset) UtechSmart USB 2.0 to 10/100 Fast Ethernet LAN Wired Network Adapter for Wii, Wii U, Macbook, Surface Pro,Chromebook, Windows 10, 8.1 and Earlier, Linux

That's what my mom bought.

Configure the portable router in client mode. First you connect to it by WiFi, the network name and password are on the router itself. When you are connected to it the address you enter is "tplinklogin.net".

After you change it to client mode, you can connect to it by ethernet. You might need to turn your WiFi off. If the internet doesn't work, click network in the web configuration, and change it from smart to static, then save it. Most likely still won't work, so change it back to smart, and then wait for it to reboot again, and it should work. Rebooting might fix it too.

The username and password for the web configuration is admin for both the username and password.

I tried playing Splatoon, the first time it said the other person had an internet problem, the second time it worked, it stayed connected for the whole game too.