Topic: Can't connect! :(

I am trying to connect my 3ds to the internet to play pokemon ultra moon online. I scanned and found the router, entered the right password and it says "testing connection" for about 5 minutes and then fails. I am really wanting to trade pokemon so I am asking anyone knowledgeable for help. I know nothing about wifi or 3ds consoles or connection issues. Like literally nothing. If anybody could give me suggestions/troubleshooting in simple steps and terms I would be extremely grateful. And if anyone else is also wanting to play pokemon we can do that too. After I get this fixed...

@Woomy_NNYes Thank you for replying. I should have also mentioned I have connected this system to the internet before about 2 years ago. But I only had one game and stopped playing 3ds since then until now, and it no longer works. So it was working without ever hearing of nintendo.com. What does this mean? I'm gonna look at the website but I don't really know what I'm doing, particularly with the "link your 3ds to that account".