fx (It's a file manager app for Android) can connect to the 3DS for sure.
Just open fx, go to "Network", add a new connection, for "Server Type" choose "FTP (No Encryption)", for the "Hostname" put your 3DS' IP (WITHOUT a port!), you can leave the "Display Name", "Default Path" and "User Name" blank, for "Authentication" I have "Ask for password each time" (no password is needed when connecting, so it doesn't even ask), under "Advanced Options" check "Use alternate port" and put "5000". Also uncheck "Use passive mode for connections" at the very bottom. I don't know if the settings have to be exactly like this, but it works for me.
If it still doesn't work try using FTPD on your 3DS instead of the built in SD Management.