Unreinforced masonry (URM) buildings, many of which are non-engineered and lack seismic detailing, remain highly vulnerable to earthquake damage. To address this, a retrofit solution was introduced that utilizes modular steel frames to enhance the out-of-plane stability and in-plane capacity of URM walls. The performance of this solution was evaluated through an experimental campaign conducted at the EUCENTRE Foundation in 2020, followed by numerical simulations using a simplified micro-modelling approach within the Distinct Element Method (DEM) framework. In this approach, brick units were represented as hexahedral blocks with nonlinear behaviour, while mortar joints were modelled as zero-thickness interfaces. The retrofit frames were included using linear-elastic beam elements and non-linear links to simulate anchor connections. The present study also assessed an alternative modelling strategy within the DEM framework, which employed an interface-based model incorporating shear-compression coupling with hardening and softening laws. The results of both strategies were critically compared in terms of simulation accuracy and computational effort, and a sensitivity analysis on material parameters was also performed. While the comparison highlights the potential of both DEM strategies to simulate the seismic response of URM and retrofitted walls, lumping all nonlinearities in the joints yielded a slightly more accurate prediction with reduced computational effort.