You said you were using a forwarder, were you trying to connect to wifi in 3DS mode or DS mode? DS mode wifi slots are separate from 3ds slots, and have to be set up under Internet Settings -> Nintendo DS Connections.
If that's not it, the problem's probably with the network itself. To check, we'll need another access point to connect to. If you have an android phone, you can host a hotspot over mobile data and connect to that. Windows also supports hosting access points, though you'll need ethernet for that to be helpful
If things work normally using your hosted AP, there's some sort of problem with your regular wifi network.