I have hotspot NAS at different geographical locations centrally managed by CHR RADIUS (Userman) hosted at Amazon Web Services (AWS). The NAS connects to CHR RADIUS through PPTP. AAA works fine, however, the user(s) are not automatically disconnected from NAS as soon as the time set on profile limit...

For your first How to:
Make sure in the RADIUS (Userman) profile, the number of shared users is limited to one.