If the light is not purple, then your Wii U probably does not have ISFShax. I guess the only way to fix it is by using
UDPIH as SDIO said, sadly it will require you to use another device, which needs to be one of these:
- Raspberry Pi Pico (W) / Pico 2 (W)
- Raspberry Pi Zero (W) / A / A+ / Zero 2 W / 4 / 5
- Steam Deck
- Nintendo Switch capable of running udpih_nxpayload
Note: Any other linux device capable of USB device emulation should work as well.
(This information has been taken from the official
UDPIH page).