In the past, we successfully got some per-app data usage reported with Android+ report "Device Reports(Flat View)->Telecom Data Usage Data Collection" on v13.x implementations.  I think we had TEM rule and/or data-collection rule configured to collect the required data from the devices of interest.  I still have screenshot of the report with upload/download data entries for individual apps.  However,  I can't recall which server/agent versions were used in our cases, nor the device brand/model and firmware versions (likely to be android 5.x or 6.x)

I don't know which server/device-agent versions you are using.  You can try that out yourself.  Beware that you need to enable "usage data" permission for the device agent on the device's Settings's app-manager tab.  Google seems to put more and more restrictions in recent/coming Android firmware versions  (i.e. 7.x+) on whether a non-system app  (including MobiControl device agent) can access such sensitive data related to end-user privacy.  So, you might have difficulties achieving what you want in your current situation, or may be in the near-future when the device firmware is upgraded.   Good luck.

Do you want to get detailed data usage for each app or just the total data usage? 

If you just want to threshold the total data usage for a device to initiate policy change(s) or other action(s),you can simply use Telecom Expense Management rule to achieve that.

Basically we need to breakdown usage as much as possible, so by app would best. Essentially the data displayed in the 'data usage' tab in settings on the device.