With the release of IOS 12, the instructions for adding an Office 365 account to the default iPhone App have changed slightly from IOS 11.  Here are the revised instructions.

Step 1 – Click on Settings from your iPhone Home Screen

Step 2 – Click on Passwords & Accounts

Step 3 – Click on Add Account

Step 4 – Click on Exchange

Step 5 – Enter your Email Address, Description and click on Next

Step 6 – Click on Sign In

Step 7 – Enter your email address and Office 365 Password

Step 8 – Click on Save Password and then Accept

Step 9 – Click on Save