Open XChange certificate

Hello folks. I hope everyone is having a great day!

I have OpenXchange using Let’sEncrypt… https Web works fine, it shows Let’s Encrypt as the SSl cert issuer and is trusted. The problem is when I try to add an account to iPhone, the certificate is untrusted because it shows the cert provider is Univention.

How can I get IMAP to use the Let’s Encrypt cert?