Me88, are you still having problems with this? I may be able to help.
App for Calender & Tasks
What app can you recomand for syncing nextcloud Calender & Tasks
Is there an App that have both possibilities?
Best an App which is available for Android and iPhone
Hope for Help
The only app we know supporting both is acalendar+, because they implemented “Open Tasks”. I don’t think they offer an iPhone version.