If you run the Nextcloud app on your mobile device and try to open an office document, spread sheet, presentation, etc., you will not be able to edit that document.
Unfortunately, this is a limitation of OnlyOffice.
However, you can download the OnlyOffice app (android or iPhone) and configure it to connect to Nextcloud (using the server address: https://share.mayfirst.org/). Then, you can edit those documents via the OnlyOffice app.