@1962up @ldexterldesign
The iOS app is compatible with iCloud Drive, Dropbox, Google Drive, OneDrive, pCloud, as well as WebDAV and S3 cloud storage. I believe the desktop app will work with most cloud storage services that have desktop sync software (which the above services offer) assuming you install them on your computer. The Cryptomator desktop app needs a “local” storage location to work with, which is what the cloud desktop apps provide.
BTW, Telekom has a MagentaCLOUD app for Windows and MacOS. I haven’t tried it (I’m no longer a Telekom customer), but it may be compatible (at least on the desktop). Best of luck!