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.