Hello Alvaro,
please use the service manager of the source BO (that has the cross-bo association and the representation node defined). You can receive the instance via the factory:
data lo_sm type ref to /bobf/if_tra_service_manager.
lo_sm = /bobf/if_tra_serv_mgr_factory=>get_service_manager( iv_bo_key = <CONSTANT_INTERFACE_OF_SOURCE_BO>=>sc_bo_key ).
lo_sm->retrieve_by_association( ... ).
Attention: Delegated Nodes and Representation Nodes are different kinds of nodes. A delegated node includes a dependent object, that means it is replaced by reuse logic. A representation node is just a reference to a node of a foreign business object.
Best regards
Tilmann