Nintendo Switch: Mario Kart 8 Deluxe - Error Code 2618-0516?

For a few months now I have had the problem that I get the above error code on Mario Kart 8 Deluxe (Nintendo Switch) when I want to race online in global game mode.

I'm getting desperate because I've already tried a lot: turning the router off and on again, uninstalling the game and reinstalling it, I (or my friend, I can't do that) have already tried to open a port for the switch using this Videos ( set up but either I was unsuccessful or it didn't work. I also asked two friends of mine who also own a Switch, but one of them doesn't have this problem and the other person has never played in this mode. The online function for other games (e.g. Animal Crossing) has always worked for me so far.

My last "hope" now is to write to Nintendo Support. Did any of you have the same problem and have been able to solve it? I'm grateful for any help!

You may need a public IP, ask your internet provider about this.

For someone else who had exactly the same problem, it was solved that way, according to the A1 forum.
Link to the forum post: https://www.a1community.net/gaming-gear-gaming-modem-418/nintendo-switch-online-game-problem-wegen-nat-c-d-250407