To push down an app to iPhones or iPads you need to "Purchase" the app through the Apple Volume Purchase Program (VPP)
Sign in to business.apple.com and find the app under apps and books. Purchase enough licenses for the need. Don't purchase thousands, this can cause sync issues.
Once purchased open endpoint.microsoft.com and go to Tenant Administration on the left.
Select Connectors and Tokens. Select Apple VPP Tokens. Right click on the Apple VPP token and click sync.
The new app should now appear in the Apps section. You can modify app assignments under Properties. Assign it to a group (or all devices). Make sure the license type is "Device".