All we can really recommend about NAT Types is reading this following thread, or contacting your Internet Service Provider (ISP) to make your NAT Type better.
[Detecting NAT type on Wii U and 3DS (Guide)]
Before I begin, I would like to thank @piplupness and @PN_quarky for helping me test this, as well as some initial information posted by @numerofoxtheglaceon_ on Pretendo’s Discord server.
I initially started working on this after information was very vague on how to decode a code that the Connection Test outputs, all I had to go from when I started was a message on a Wii homebrew forum.
This is by no means complete yet, we still don’t fully know…