Android - How to solve the "APN settings are not available for this user" issue?
After hours of experimenting, found the reason for this, (it works for me). Nothing too technical about this post in contrast to what these posts suggest.
Step 1 : WiFi turned on, and be in the range of a known WiFi network.
Step 2 : Reboot the phone (may need to reboot several times, if it doesn't work the first time)
Step 3 : When the phone reboots, it should connect to WiFi network.
Step 4 : Go to APN settings and see that you can, now edit APN settings.
Reason : When phone is turned on, if it's not connected to WiFi, it will try and connect to mobile network thus disabling you from editing APN settings. However if you try the above steps, your phone connects to WiFi instead of Mobile-data.
As you have root permissions, try copy and replace telephony related files from CM12.1 to your phone.
/data/data/com.android.providers.telephony/databases/ is the path where telephony.db exists
Note: This may harm your normal working of your phone; try it with your own risk.
The latest stock ASUS update (v. 3.24.40.78) solves the problem.
I had to unroot my phone, apply both 5.3.3.3 and 5.2.2.2 downgrades (in this order), then I applied a FOTA upgrade (97Mb download, received and applied automatically). Then I could install the latest version (downloaded from ASUS website; uploaded via USB to phone's internal storage; unplugged USB and then the update could be applied automatically). APNs are available by default, I did not even have to input my APNs manually (as I had to do in prior versions).