You can either: A) Clean the button membranes with the help of this guide: Button Replacement Guide B) Use a paperclip to press the small reset button on the back of the controller located on the top right of the label C) Follow this video if the previous options don’t work. If none of the solutions work, then I suggest taking them to a repair center or buying new ones. Hope this helps!
The Wii U never had GCN compatibility out of the box, and the only way to play GameCube games on it is to homebrew the Virtual Wii (vWii) and use Nintendont with a USB flash drive that has GCN backups on it. You can find multiple YouTube videos on how to do this. Hope this helps!
For people wondering: Yes, this does work on a MID7120, because the MID7120's board is similar to the MID7125's board. Also, where can I get switches similar to these? My MID7120 is missing the switch so I can't turn it on.