rclone in the other thread was a suggestion for cli2webdav - but for phone - if the inbuilt eDrive isn’t flexible enough (because of what folders to sync), the official nextcloud client has that configurability and is a solid choice.
Another condition is allowing client-sync in the nextcloud settings? - Problem with file sync after service-outage - #2 by tcecyk
As for RoundSync - I couldn’t get got it to work in the version 2.5.6 with a rclone.conf import that is proven to work with rclone itself - but I changed vendor to “other” instead of “nextcloud” (because this issue comment). What I was also missing was creating a task that actually setups a sync.