Is it possible, that the splatoon server is down these days?
The Nintendo Network service, that handled online play for the vast majority of 3DS and Wii U games, Splatoon included, was shut down on the 8th of April. So yes, the official servers for Splatoon are dead.
I think because you appear to clarify something that was seemingly clear in the first post. But if English isn’t your first language that’s understandable.
Checkout https://pretendo.network to use the open source replacement for Nintendo’s servers
Yes, use the SSSL (hackles) setup: https://pretendo.network/docs/install/wiiu#sssl
This doesn’t require homebrew, all you need is changing the DNS address on the WiiU from the system setting.