Well that's not encouraging. I haven't tried it yet so I'm not sure how to troubleshoot that one. I had just seen these announcements over the last few months:
https://nextcloud.com/blog/collabora-online-developer-edition-4.0-is-here-edit-documents-on-mobile-and-more/ (article is 2 months old now and the regular release of 4.0 looks to have been released last week)
Nextcloud's app supposedly already has support for it.