[Zubersoft]

If you already backed up your library, uninstalled the Microsoft Store version, installed the FastSpring version, entered the license key and restored your library, you can then go to the Windows Settings->Apps->Installed Apps->MobileSheets, click the three dots and Advanced Settings, then just toggle on "File system". MobileSheets will then have access to the entire file system and should be able to open files from any location.

Mike