Valid purchased S/MIME from Digicert unable to encrypt in Iphone v- iOS 12.1.4

Hello, I can send and receive encrypted emails from iMac and I can read those encrypted mails in my iPhone. However, the iPhone does not allow me to send encrypted messages.

I followed the instructions to configure SSL on and selected the private certificated for signing and default encryption with the certificated selected.

I also tried to remove all profiles and reinstalled and tried going to General - about -> Certificate Trust Settings but I do not have further options for configuration.

Any tip that you can give me to have the mobile phone work properly?

Thanks