Here are the steps required to setup Microsoft Office 365 on iPhone / iPad
Go to Settings > Accounts & Passwords and tap Add Account
In the Add Account page, tap Exchange.
Enter your full email address, for example email@example.com, and your password, and then tap Next.
By default, Mail, Contacts, and Calendar information are synchronized. Tap Save.
If you're prompted to create a passcode, tap Continue and type a numeric passcode. If you're prompted and don't set up a passcode, you can't view this account on your device. You can also set up a passcode later in your settings.
If your device can't connect make sure you typed your email address and password correctly and try again.
If you require access to some of the enhanced functionality, such as: Calendar sharing etc you will need to download and install the Outlook OWA App.
You can get the app from the app store at: https://appsto.re/au/3MZtN.i