LDAP authentication and DB entity user for roles

I'm using Symfony 4(.4.11) and trying to implement a LDAP Authentication, while the User Properties (Username and Roles) are stored in a DB. Thus I added a User Entity for Doctrine and configurated the services.yml and security.yml corresponding to the Documentation.

I also used the Maker Bundle to generate a LoginFormAuthenticator which seems to use the Guard Authenticator Module.

When I'm trying to login it simply looks like it is not doing anything LDAP related. I also listened the TCP packages with tcpdump and didn't see any traffic to the LDAP server.

I have read (and re-read) the security docs but this in particular:

https://symfony.com/doc/current/security/ldap.html

that said :

The http_basic_ldap authentication provider, for authenticating against an LDAP server using HTTP Basic. Like all other authentication providers, it can be used with any user provider.

Sounds like it should be possible but I haven't been able to figure out how to wire this in security.yaml and services.yaml

Currently, I can connect very well with the http_basic_ldap but how to match the username with the one in DB in order to get its roles?

Thoughts?