SFTP and rsync do not work on Ubiquiti radios. When you attempt to SFTP, you receive the error:

sh: /usr/libexec/sftp-server: not found

We can however use SCP with the -O option, which tells SCP to use the legacy SCP protocol, for file transfer instead of SFTP. The following command will download the radio config.

scp -O ubnt@192.168.1.20:/tmp/system.cfg ./

If you want to download an entire directory, or the entire OS for that matter, you can use the -r flag for SCP. The following command will download all the files in the /tmp directory.

scp -O -r ubnt@192.168.1.20:/tmp ./