No wifi connection on DS Lite

I recently switched internet providers (Telenet, in Belgium) and I want to be able to connect my DS Lite to my new router (which they also provided me with).
It's a "CPN / SVG6540E" combined router/modem with wifi and EURODOCSIS 3.0 technology.

When I add a new connection in my DS Lite (through a game with wifi), I'm able to see the SSID of my router, which shows on top, meaning it has the strongest connection from where I'm standing. But when I click on it and start a connections test, it gives me either error 51300 or 51302.

I already checked out the Nintendo error guide, which tells me that the password I entered on my DS Lite isn't the same as the password of the router. I had set it to WEP, because it can't handle WPA yet.

So what I did was turn off security of my router altogether, thinking that would at least clear that error message. But no go. It still gives me error 51300 or 51302.

I'm at a loss here, and there are some wifi events coming up soon. What am I doing wrong?

update 1:
I'm fairly positive it has something to do with the router, more than with the ds lite, because I can connect to the network at my parent's home without a hitch. I also just tried to connect my girlfriend's ds lite to the network here, also without any success.
I've already contacted the ISP and they told me that, since I can connect perfectly fine with other devices, there shouldn't be any problem..

update 2:
I just tried connecting with my wii, just to be sure other nintendo devices could connect. Connection was successful.