Calendar invite not showing up on recipient phone

My partner recently got an iphone, and we're trying to set up a shared calendar. We both have apple-ids with \@gmail.com domains, but use the apple calendar app. When I create a calendar, and try to share it there is no way for her to accept it. No notification in the calendar app, no email, no popup on her phone. Finally when checking the calendar tab on the icloud website did we find the notification, but when accepting it still didn't appear in the mobile calendar app. How can we get this working?