Further to the post above mine, if you have a Carrier Update (which means in your Settings/General/About), and it shows the provider (instead of Carrier), then you won't have that option if you go to Cellular/Cellular Data Options.
If you have Reliance/Jio as your provider, then you will not have access to 3G. Apparently from what I read, they built a LTE network from scratch. On my provider, I have a toggle switch to toggle LTE on and off. If I toggle it off, it's on 3G for everything, including data. 2G networks in Canada have been decommissioned, and allocated for LTE.