Try "ES File Explorer" and make sure to turn off their annoying screen lock. The network option in that has a FTP server that I know for sure works with the 3DS.I've tried a ton of different apps and only one app sees and connects to my 3ds and then doesn't see any files
OP has already said he has tried several apps that do not work. It would be more helpful if you could name apps that you have yourself confirmed to work with the 3ds. I was also having the same problem as OP until I tried the app I mentioned, but if people know any others that work for sure, it would be appreciated. ES is kinda annoying....If you have homebrew, the ftp app for 3ds and an ftp app on your phone, works just fine.
Enter the IP and port, then enter literally anything for the username.I tried es file explorer it asks for a username and what would the hostname be I've tried the IP and I've tried 3DS-****
AndFTP that's the app I used. Worked just fine. Could transfer files from 3ds to phone and from phone to 3ds. Hope it works for you.
It might be a firewall blocking the connection to port 5000 on a ftp server
I tried andsmb earlier it connects but it can't see any filed
Are you using the n3ds built in sd manager? Or a homebrew FTP client? Try a homebrew version if that's not what you're already doing. That's what I use. Never tried the built in one.