Depending on your company's security settings, you may need to Using the Company Portal App on iOS in Order to Access Microsoft 365 Email first.
1. Download Outlook for iOS from the iOS App Store and then open it. The icon looks like this:
Note: See system requirements for your iOS devices here.
2. If it's your first time using Outlook for iOS, enter your full email address, then tap Add Account.
If not, open the Menu > tap Settings > Add Account > Add Email Account.
Also see Microsoft's article: https://support.office.com/en-us/article/set-up-office-apps-and-email-on-ios-devices-0402b37e-49c4-4419-a030-f34c2013041f?ui=en-US&rs=en-US&ad=US