Ledger Nano.

As you can pay for an iOS, i strongly recommend buying a safe hardware wallet for your coins.
This is the best wallet for iOS.

If you do not want the best, there are few options.
https://medium.com/@rogermagno/segwit-wallets-02-07-2018-76d836bccbc9

None of them are very good.... I would keep my coins in a more trusted legacy wallet like blockchain.info until buying a ledgernano.