I use PGP/GPG to handle email encryption on the PC and Android (via APG). Surely there is a PGP/GPG client for IOS? Forgive me, if not, as I don't own an iPhone.
Take a look at iPGmail
in the app store.
Can't vouch for either since I don't trust my iPhone to begin with. It's a proprietary black box - so any sense of real
personal data security you may have is either wishful thinking, or misplaced hope at best.
About the only "signal security "that's still workable is a single-use cipher pad used in conjunction with a piece of paper delivered by a human courier. If a message truly
needs to be kept secure - keep it off the grid. Completely