As explained in this post, I managed to create a first communication with Truenas, but Truenas is having some issues.
Let me start from the begining. I followed the PAM configuration guide, to have a base server, and LDAPS is working.
I did not configure a specific UID value though, same for GUID as my first goal was to have the system establish a connection, and accept this server. Moreover, I also have to admit I don't really know what to set these values to.