The main result of this paper establishes a bijection between the set of equivalence classes of simple transitive 2-representations with a fixed apex J of a fiat 2-category C and the set of equivalence classes of faithful simple transitive 2-representations of the fiat 2-subquotient of C associated with a diagonal H-cell in J . As an application, we classify simple transitive 2-representations of various categories of Soergel bimodules, in particular, completing the classification in types B 3 and B 4 .We also thank Tobias Kildetoft for his help regarding the computation of cells in types B 3 , B 4 and B 5 .2. Preliminaries in abstract 2-representation theory 2.1. 2-categories and 2-representations. We refer to [Le, McL] for generalities and terminology on 2-categories. We refer to [MM1, MM2, MM3, MM4, MM5, MM6] for