It is possible to use SimpleBackups to back up private resources. You will need an intermediate server within your network that has access to the private network, and use that intermediate server during the backup.
Using this method, the private host is never accessible or exposed to the Internet, and at the same time no agents are run on the private server to take the backup. The private host stays as a blackbox.
Private database, or server you need to back up (ex.:
Any SSH-accessible intermediate server, that has access to the resources in step 1 (ex.:
Back Up Internal or Private Databases (database backups)
Expose the private database that resides on
private-resource-01 to an intermediate SSH-accessible server
Create a serverless database backup on SimpleBackups
Use the tunnel option to tunnel SimpleBackups' connection through
tunnel-server-01which has access to your privately accessible database