Restore active exchange sync using third party apps

The Outlook app is a bit different in that if you sync using that it won't be possible to see the calendar in third party apps.

You'll need to use Exchange as the sync provider. Some apps that can do this are:
https://play.google.com/store/apps/details?id=it.mp.calendar.sync&hl=en
https://play.google.com/store/apps/details?id=com.ninefolders.hd3&hl=en

Then you need to go to phone settings >accounts > add an account > nine > enter your details there. (You should decouple the Outlook account first before doing this).