Achieving in-situ control over the spectroscopic characteristics of strongly coupled systems, such as the polariton frequency and linewidth, holds the potential for investigating new physics and developing novel polaritonic devices. Here, we study the effect of a applying a perpendicular magnetic field on ISB transitions strongly coupled to dispersive metal-insulator-metal (MIM) ribbon microcavities. We probe the system using terahertz time-domain spectroscopy (THz-TDS) in a reflectance geometry. The dominant mode in the microcavity is polarized perpendicular to the quantum well plane and efficiently excites the ISB transition. However, the cavity also partially couples to the orthogonally polarized cyclotron resonance, leading to a pronounced interaction between the ISB lower polariton and the cyclotron resonance, which can be tuned with the magnetic field strength.