This paper presents a novel dual-band cylindrical dielectric resonator antenna featuring a distinctive cross-ring structure that enables circular polarization (CP) across two critical frequency bands. The proposed antenna operates efficiently in both the 8–16 GHz microwave band and the 24–30 GHz millimeter-wave band, making it particularly suitable for ultra-wideband (UWB) and emerging 6G applications. Examination of the $S_{11}$ parameters reveals excellent impedance matching below $-10$ dB, yielding an impressive bandwidth of approximately 8 GHz (64.26%) in the lower band and approximately 6 GHz (23.72%) in the upper band. The antenna achieves stable CP through degenerate orthogonal modes, i.e., $\mathrm{TM}^{x}_{11\delta}$ and $\mathrm{TM}^{y}_{11\delta}$, in the cylindrical dielectric resonator, with 3-dB axial-ratio bandwidth extending across both operating bands (approximately 8 GHz in the UWB band, representing 54.91% bandwidth, and approximately 6 GHz in the mmWave band, representing 20.12% bandwidth). The antenna delivers consistent gain performance, averaging 7.0 dBi in the UWB band and 6.5 dBic in the mmWave band. The novelty of this design lies in its elegant cross-ring feeding structure that simultaneously enables dual-band operation, excellent CP characteristics, and high radiation efficiency in a compact form factor. Simulation results closely align with measured data, confirming the antenna’s suitability for next-generation wireless communication systems.