Q: how do I share internet between my iphone 4 and my ipad?

I have an iphone 4S (model: MC603B/A) with 7.1.2 (11DD257). The operator is Carrier 16.1.

I have tried to share internet between this iphone and my ipad, but I cannot do it because when I arrive at the relevant option through adjustments (mobile data>set up "share internet"), I always get the same message: (to activate "share internet" in this account, contact Carrier - translation from Spanish.

I have contacted ONO, which is the phone company that is providing me with service, thinking that it was my "carrier", and they sent me back to "apple" to sort this out. They say that I could have this service if my iphone was well set up but they cannot do it themselves.

How could I set it up?

Thanks for your help in advance,

Telma