We have signed an Enterprise Agreement with Microsoft. May our employees use Office 365 ProPlus licenses on their private devices?

Yes, you may use an Office 365 ProPlus (user) license purchased under Enterprise Agreement on up to 5 computers. In this case, it does not matter if the computer is used for personal or commercial purposes.

Source: Volume Licensing Brief_Microsoft_Office ProPlus_as is 2017-08-16