Start here (en español): Usar la red 5G con el iPhone - Soporte técnico de Apple (CL)
The Apple list of supported 5G carriers include Entel (en Español) : Soporte técnico y funciones de los operadores de telefonía celular para iPhone en Latinoamérica y el Caribe - Soporte técnico de Apple
This 5G network is all fairly recent, too: Ericsson got a contract with Entel to deploy 5G back in June of 2021, and President Piñera and cellular providers announced the start of 5G on December 16th (last week), and there is apparently just now a free promotion being offered.
On the website, Entel lists compatible devices, which does not presently include Apple: https://www.entel.cl/5g/catalogo%2D5g/
As for turning on 5G, that’s set when the carrier propagates an update enabling that, and when there is a 5G network available.
I’d guess the necessary carrier update for Entel 5G is stuck somewhere, but it sure looks like this should work.
And to be clear, we’re not folks that work for Apple, we’re other users like you. We have no inside information on Apple, on 5G support in Chile, nor on Entel.