Understand that this is an older thread, but I was working with the AOI (Area of Interest) feature and I may have figured this one out... Here's the step-by-step of how I did it for everyone's collective benefit:
a) Clone the camera you seek to define the AOI on.
b) On the cloned camera settings, delete all the streams, except for the main stream (do this on the Video tab ---> click on Configure --> change the Sub stream to "none", leaving everything else alone.
c) Click ok and then ok again. The cloned camera will restart/reboot.
d) Now get onto the settings of the cloned camera once again and onto the Video tab. This time around the AIO will not be greyed out. Check the AOI option and click on EDIT to set the AIO you need/want.
e) Once you are happy with your AOI, then click ok twice and you'll be all set.
Note: you will not be able to further edit / change the AOI from there onwards, as it will be greyed out again. If you want/need to tweak the AOI again, you'll need to re-clone the camera and start over (which is not a big deal as the process is quite simple).
Hope this helps!