Depends what you mean by security. iPhone is already secure assuming your device is fully updated.

If you mean privacy, there's little you can do if your iCloud account is tied to your identity. Apple knows everything about your phone usage regardless of a VPN.

If you want to hide your IP from Nostr relays a VPN would do that. Mulvad, Proton & IVPN are what most mention.

I personally wouldnt bother with Tor unless your going full on anon, creating a new nsec and willing to put up with a slower than expected performance forever.