I have generated public certificate with CN=FQDN of domain server, there is also key extension in certificate with: server auth (OID: 188.8.131.52.184.108.40.206.1), certificate CSR was done on domain controller then imported newly issued certificate into computer account certificates.
Then I have imported also CA_root certificate to Fortigate.
As I told from my pc when use application like lpdadmin I can connect to FQDN of my domain controller on port 636, I then confirm this on domain controller by command netstat -an | find ":636" that connection is established. If I choose IP address on lpadmin instead of FQDN domain controller, then I cannot connect on 636 port, so I think this provide that LDAPS is working correctly.
But on Fortigate side, when connecting using secure connection with 636 port, I cannot connect.