OnlyOffice is a part of the e-cloud service.
I wold like to access the files I made in the browser with my smartphone and desktop apps. How do I do that?
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I use collabora office to open and edit office files from the coud, mostly excel docs. You can get it on the Apps library. It is based on Libre Office.
It works pretty good. Runs a little slow on the terracube, but sufficient for my purposes.
The other option is to just bookmark the page to your files on /e/ cloud and edit them through the browser. That works ok too.
There is an android client for OnlyOffice in the App Lounge (OnlyOffice Documents APP). Just install, connect your account and be happy 
…at least I’m with the solution.
OnlyOffice also supports desktop clients. In Linux they support provide a flatpak version.