A dielectric resonator is shielded by a PEC cavity, and thus eigenmode simulation in Ansoft HFSS will acquire resonant modes of both the metal cavity and the dielectric resonator. if you only want the resonant frequency of dielectric resonator, you must have a cavity with dimensions at least 2 or 3 wavelength larger than the dielectric resonator. then set the lower freq. of eigenmode solution near ( a bit lower) the lowest mode of dielectric resonator.