Thanks for the reply.
Sorry for not specifying earlier. I am using iPhone 4s and not iPad. I tried the solutions given via google that is:
1) Reset the network settings and reboot
2) Rebooting with the wifi disabled.
But still to no avail. The personal hotspot option is no where to be found.
I also have no option to set the APN as I also cannot find this anywhere. (Also not found in Settings -> General -> Network -> Cellular Data)
At the Cellular Data, I only have option to Off or ON and no other settings. 😟