Unlike Staff iPads, the account does not automatically sign you into Office.
To access the apps, click on an app (Office, OneDrive etc)
Click Existing Microsoft 365 Users? Sign In
The email should automatically be entered - so just enter the password.
(If the email hasn't, simply enter it - followed by the password).
Select Sign in
You will then be signed in to all of the Office apps automatically from now on. You won't need to sign in again.
Furthermore, all Microsoft Cloud services are automatically available (such as OneDrive). So any existing documents can be opened (see picture, below) and any new documents can be saved.
It is also possible to copy and paste between iCloud and OneDrive by adding OneDrive to the FILES app.
After signing into an Office app, open OneDrive to make sure it's logged in. Then open FILES.
You will see a red '1' on the left, underneath My Files. Select it to add OneDrive to the list.