Urtext is local first. Reliance on cloud services is not just outside its scope but avoided by design.
Not always stated in user-facing documentation is that cloud-based file applications such as Dropbox, Google Drive or iCloud Drive do not retain or sync local copies of all files at all times. They also lack a means to directly initiate file sync or a robust method to resolve differences across devices.
The recommended solution is to store Urtext files in a distributed version control system such as Git.
The recommended client for iOS is Working Copy.