I set up a small office with Office 365 about 2 months ago, including a sharepoint calendar.
The calendar works great via a web site or through outlook & up until a couple of weeks ago, through the Sharepoint app for IOS.
All users have iphone 7, IOS versions 11.2.2, 10.3.3 & 11.2.5
The calendar just does not show up anymore in the mobile app.
I have deleted & reinstalled, but nothing.