If you have the access to Plesk Control Panel, here is the best way to setup your email on your iOS devices.
https://docs.plesk.com/en-US/obsidian/administrator-guide/website-management/quick-start-with-plesk/set-up-mail-accounts/2-access-your-mailbox/access-from-iphone.75203/

This will show you how to "ADD ACCOUNT" to Mail App on your iOS devices.

In case of having problem with Plesk expired certificate, please go here to solve it.
https://support.plesk.com/hc/en-us/articles/12377286818071-Mail-users-with-iOS-and-MacOS-devices-cannot-access-mail-after-certificate-renewal-on-Plesk-server-Cannot-Verify-Server-Identity