Stratum does not have permission to access the Internet and does not implement any Cloud sync functionality.
If you wish to save your Stratum backups to a WebDAV server, the "Auto Backup" can be used in conjunction with DAVx5 to enable this.
DAVx5 is a FOSS WebDAV tool that allows you to synchronise contacts, calendars, TODO lists and, allows you to create WebDAV mounts.
Step 0: Download and install DAVx5
DAVx5 can be downloaded using any of the official channels (Google Play, F-Droid, etc.):
https://www.davx5.com/download
Step 1: Create a WebDAV mount
Open DAVx5 and open the menu, go to the "WebDAV mounts" page.
Step up the mount using your WebDAV server information and credentials.
Step 2: Configure Auto Backups in Stratum
In Stratum, open the settings and select "Auto Backup". Select a backup location and in the file menu, there will a WebDAV mount entry. Select the desired folder, choose "Use This Folder", and allow permission to access files.
Step 3: Perform auto backups
Enable auto backups with the location and password of your choice. Stratum will automatically write backup files when important changes are made. DAVx5 will be notified of a new file, and will synchronise it with your WebDAV server.