This paper addresses some critical challenges in the design of multiple-input multiple-output (MIMO) antennas, namely mutual coupling, size constraint, and diversity by introducing a compact metamaterial-grounded ultra-wideband (UWB) 4-port MIMO cross-fractal antenna tailored for K, Ka, and millimeter-wave (mmWave) applications. The proposed 5G planar antenna leverages a hybrid structure incorporating coplanar waveguide (CPW) and metasurface elements, implemented on 11.82 × 11.82 × 0.35 mm3 Rogers RT5880 substrate with a dielectric constant of 2.2. The MIMO antenna architecture is derived from a single-input single-output (SISO) UWB antenna design, serving as the foundation for the realization of the MIMO antenna frontend. The four-port cross-fractal antenna elements are strategically positioned orthogonally (900) on top of the substrate, featuring a double square split ring at the substrate bottom. The resulting antenna exhibits an impressive operating bandwidth of 23.65 GHz (17.75–41.40 GHz), effectively covering the K, Ka, and mmWave bands. Notably, the envelope correlation coefficient (ECC) for the entire bandwidth remains consistently below 0.0013, showcasing the antenna’s robust performance. Furthermore, the diversity gain of the antenna within all operating bandwidths surpasses 9.99 dB. The significance of this MIMO antenna lies in its unique combination of ultra-wide bandwidth, small form factor (SFF), low inter-element spacing (2.5 mm), and exceptional ECC (<0.0013), positioning it as a noteworthy contribution to the field.