In March 2020, Microsoft is going to release a update which will essentially disable the use of unsigned LDAP which will be the default. This means that you can no longer use bindings or services which binds to domain controllers over unsigned ldap on port 389. You can either use LDAPS over port 636 or using StartTLS on port 389 but it still requires that you addd a certificate to your domain controllers. This hardening can be done manually until the release of the security update that will enable these settings by default.
https://msandbu.org/upcoming-change-microsoft-to-disable-use-of-unsigned-ldap-port-389/
Therefore App Portal 2019 or earlier doesn't support AD LDAPs integration.
By the time of this KB is written June 16th 2020. We have ETA to have this capability in 2020 release
