I had a hard time trying to understand how to use GPG >= 2.1 with an LDAP server, as the keyserver option binddn was not recognized anymore.
The documentation page I found is this one: https://wiki.gnupg.org/LDAPKeyserver
But it is far from complete. I managed to push to my LDAP server using the syntax found in this old email: https://lists.gnupg.org/pipermail/gnupg-devel/2015-March/029607.html
This syntax should be stated in the documentation if it’s the correct way to push to a protected LDAP server.
Then I also had the problem stated in https://dev.gnupg.org/T4447 but it seems it’s a bug with a patch waiting inclusion (I wondered at first if this was a new behavior on purpose).