Configuring Synology NAS as freeIPA client
Solution 1:
Clients (especially MS) look up DNS for LDAP. DNS must have a srv for LDAP.
dig [@ns] _ldap._tcp.example.com srv
might reveal if it has one. If the NAS still can't find it, adding an MS-specific one might help, like
_ldap._tcp.dc._msdcs.example.com. <ttl> IN SRV <prio> <weight> 389 freeipa.example.com.
and
freeipa.example.com. <ttl> IN A <ip address>