Here is how to rest the controller to zero the left and right stick:
https://en-americas-support.nintendo.com/app/answers/detail/a_id/1107/~/how-to-reset-the-l-and-r-control-sticks-to-a-neutral-position-on-the-wii-u-pro
Try it a few times, turn off the system and try it again, then turn on the system, let it load to the menu, and try again
If that doessn’t work, i can think of two options, disconnect the battery inside for short time, then repeat the process.
Finally, i had the same problem with my wii U Main (With the screen) controller, and the controller part inside had to be replaced from playing Splatoon too much :) - i got it for $10 and did it in 5 minutes.
Try exhausting the resetting techniques first. Report back.