I've been using the FTP-GMX homebrew app to wirelessly transfer files to and from a PC to my 3DS.
However I noticed that the 3DS only seems to successfully connect to a wifi network that is also connected to the internet.
When I tried doing a similar wifi transfer through an offline router...
I've been following the 3DS hacks guide for installing a cfw using the Pichaxx method,
and successfully got up to the point of the Fredtool,
but got to a black screen when trying to access the Flipnote Exploit via "Internet Settings" -> "Nintendo DS Connections"
(in the "3ds hacks" website...