Usually I avoid writing into profiles now, but since this is a question for help, I'll make an exception.
The working solution is below. You may not like it, but it works for sure.
WiFi is dysfunctional crap in general. And the WiFi in the Wii (U) is super-crappy crap crap crap.
"I don't have anything like that in my house[…]"
*Sigh*
Once you get to know proper wired Ethernet you'll never look back. I have no idea why the majority decided that something is convenient/modern/better/… because it has no cables. Wireless technology adds A LOT of complexity to deliver suboptimal results under best(!) conditions.
The Wii (U) is known for notorious bad signal reception/sending and dislikes certain routers because of???
I know nothing about the error codes (and never got one on cable)
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"avoiding profiles?"
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I got a few too many low quality comments and closed my profile for comments. For the sake of fairness I avoid writing into open profiles.
Only thing drive me crazy is all these new horror games nowadays is its so dark, all time. I can't even play them. I understand they trying to create spooky but be creative a little, not just darkness with pop out scare scenes.
The working solution is below. You may not like it, but it works for sure.