Ok, well the reason they don't work well is because Nintendo replaced the Wifi card inside the DS Lites with a new one on NEWER Ds lites. Of course, changing router settings can work, but a new driver for this card in the Wifi Lib would be nice. So this means, updating your firmware does NOTHING.

So why it works for someones ? (I hadn't tried and i would prefer not to have to...)

Hiya; Yes, we have figured this out and the latest version of the wireless lib includes improved code to deal with the newer DSlites - I finially managed to get one to test with and have updated the code so it works.

So, dswifi lib 0.3.3 and higher will work with newer DS lites - if you want to try it out, test http://akkit.org/dswifi/wifi_lib_test.nds