For questions or bug reports, write to contact@techno-bibi.com. A reply may take a few days.
Check that notifications are allowed for the app in your device settings. Reminders arrive at 9 in the morning, a few days before an item is due. Notifications scheduled while the device was switched off are not delivered later; that is how the operating system works.
Unless you signed in for backup (My Page → sign in with Apple or Google), your data is kept only on the device. Sign in before you switch phones.
Create an invite code in the sharing settings on My Page and have the other person enter it. Once you approve the request, your repeat items and shopping list are shared.
See Delete your data.