Both Tuta and Proton have iOS apps of their own.
I don't believe either can be used inside the native iPhone Mail app, since they use encryption schemes, which aren't standard in any commonly used spec for mail apps, so no regularly available mail app can act as a frontend for Proton/Tuta, because no regularly available mail app "speaks" their encryption. (though on PC, Proton does have a Bridge application that can essentially perform the job of a translator in that case, but again, only for PC clients)