Use an LDAP editor like LDAPadmin.

Connect to LDAP server running on Synology NAS.

Host: Synology Nas server

Port: 389

base: cn=config

Simple authentication

TLS selected or deselected

Username: cn=config

password= same as root user LDAP server

After logging in

select cn=config and edit entry

add attribute or change value attribute olcTLSCipherSuite with your values.