What do you mean for “another client”? Another LDAP Software?
There’s LAM for manage LDAP
Also, consider to take a read for every ldap suppor related topic, maybe could help you a bit for troubleshoot the connection betwen sipXcom and OpenLDAP implementation on nethserver.
Don’t forgett that LDAPs use a self-signed certificate, unless than you configure Let’sEncrypt to fetch a certificate for your houst. Or unless you load a valid certificate for your server in another way…