You are trying to solve two different, but related problems, and there are discrete solutions for both.
One is a personal cloud. You need a secure place to store your shit from multiple users and devices, from multiple networks. You’ll need a mostly static IP and dyndns or your own domain, and certificates signed by a public CA/letsencrypt.
Then, you are looking for a backup application that supports rsync or sftp/scp over ssh or vpn, that is also cross compatible (Android and PC/Linux). Point this to the service above, and you are good to go.