So, I'm trying to connect to SoulSilver.

I found the access point, which I thought was WPA which is compatible with SS (I know for a fact), and it was supported since the padlock was red, not grey.

It asked for the 'WEP key' and I typed in the password.

It said it wasn't supported.

I checked the router's settings, it's WPA2, which I know is not supported by the DS.

I looked back and it said it was compatible but I have nothing to enter as a key besides the password.

... I'm confused as to what the DS is telling me.

You'll have to change it to WEP.

But I have successfully gotten to WiFi on SS through WPA. (Obviously not WPA2 but still).

But it still says that my router is compatible with my game. If I were to attempt to set this up with AC:WW then it wouldn't allow me to connect as it's incompatible.

The DSi's Wi-Fi configuration is linked with the DS game. It shows that it is compatible through the DSi on the game, but if you were to put the game in a DS or DSLite, it would show as a grey lock. Even though it shows as compatible, it will still not work.

EDIT: Wait a sec. So you're saying you connected successfully through SS with WPA? By that I mean you got to go to, for example, the Wi-Fi Plaza? If you did, I don't see how that's possible.

I have done it before, yes, before I forced my mom to switch to WEP so I could WiFi on WW.

Also, the DSi doesn't support WPA2 settings in the first place, I have no clue as to why it's showing as supported, which is why I came here.

It does, except you have to go to Advanced Settings.

I'm not really sure how or why it worked. I would just keep it at WEP in my opinion. WPA is not necessary at all.

My mom's house has WEP. My dad's house has WPA2 which is not working on my DSi at all, even in regular modes as in the browser etc.

The Advanced settings for a Nintendo DSi are not compatible for low Nintendo DS functions which are used for games like Pokemon SoulSilver, WarioWare D.I.Y., etc.