As far as I know, you need to be hooked into WiFi to connect, although I know that Android systems do have the capability to create hotspots. On my LG G3, if I swipe down, I can create a hotspot on the upper menu. Perhaps the 3DS could connect to that? That's a hotspot you can create for free as long as your Android has that capability.
I'd imagine it'd take up a lootttt of data if you tried to use a hotspot to go on AC:NL online though.
My boyfriend has a hotspot from Verizon that he lets me use when I dont have any connection and I absolutely need to go online with my 3ds. It doesnt take up a huge amount of data at all. That is the only way I was able to use "mobile data" with acnl.