If the option is not displayed at all, that sounds like an accelerometer issue to me (hardware issue). If your phone is still under warranty, and even if it's not, try to contact an Apple Store or an Apple Authorised Service Provider in your area first. If that doesn't help, there are also third party alternatives. :) Repair cost might differ from one option to the other. So I'll let you decide what's best for you. But try to get your phone to an Apple Store or an AASP first. See what they have to say about it. Maybe it's not hardware and I'm completely wrong. In which case, I'm sorry. But it does sound like a hardware issue to me.