Does the issue happen only with iPhones or android phones
Verify with an Android phone first.
This was an ongoing issue I think which happened with iOS 13 update.
David Favor
If you're using a Private CA this will happen for every new device, as each device has no clue about the issuance chain of a Private CA.
Fixes are...
1) You must either have each new device verify + accept the cert, each time the cert is created/renewed.
2) Just use a real cert, like free https://LetsEncrypt.org certs which work with no cert verification/acceptance step required.
David Favor
Aside: This could be another problem too.
It may be that iOS 13 update drops support for pre TLSv1.2 TLS protocols, which have all been deprecated now.
This might mean the server running (mail/web/whatever) fails to implement TLSv1.2 + TLSv1.3 or fails to enable these protocols. Most likely you just required enabling these 2x protocols + this may resolve your problem.
This is just a guess as you've provided no server name + port to test.
Verify with an Android phone first.
This was an ongoing issue I think which happened with iOS 13 update.