If sysnand and cfw are note separated, you can set the new connection in sysnand, oterwise set it in cfw booting without the L key preseend.
Don't stop trying.
I spent a whole day trying to connect with a wifi hosted connection on win7 (PC acting as a router, but without Internet connection).
My 3ds connected properly in sysnand e in standard CFW mode, but can't connect booting with L key pressed.
Then I tryed with my home router and everything went ok.
Someone said that it needs working Internet connection (only LAN is not enough?) but I dont know if this is true. Could be some difference in the lan booting sequence, not all supported.