This is from your Microsoft site!!! Outlook calendar/shared mailbox not available in Outlook IOS until the end of 2018, Why retire a fully functioning app with all the features before the other app is fully functional!! This is completely unacceptable for a service we pay for.
For the past several years, Microsoft has focused on launching features customers need in Outlook for iOS and Outlook for Android. With an architecture fully powered by the Microsoft cloud, Outlook offers an effortless user experience and bolstered security.
Many features, including shared or delegate calendar access and the ability to view the Global Address List, are now included in the Outlook apps. Other features (like shared mailboxes) will be available by the end of 2018. In the meantime, the Outlook web experience is available from your browser on your mobile device.