I spent 3-4 days running behind network carrier to enable "Personal Hotspot" option in iPhone 6s plus without any luck. Finally found the solution. If you are looking for the same, follow these steps below :
Click on settings icon : you will see following screen
Click on "Mobile Data": in the following screen shot click on "Mobile Data Network".
Scroll down till end of the page, in "personal hotspot" section, in APN input field, enter internet.
Now go back all the way to settings, again get into "Mobile Data" , you should see Set Up Personal Hotspot enabled just below Mobile Data Network Option we saw earlier.
This is where I was all lost, I clicked on Set Up Personal Hotspot option and got a message saying "To enable Personal Hotspot on this account, contact carrier", this was misleading, I kept on calling them for days, they used to come back with their standard answer.
Let's leave Set Up Personal Hotspot option as is and let's get back to the option Mobile Data Network, at the top, under MOBILE DATA section enter APN as internet
Get back to the settings screen and get into Mobile Data again, you should now see, Personal Hotspot option just below Mobile Data Network, switch it on.
Click on Turn on Wifi,
Now get back to the settings page, you should see Personal Hotspot enabled.