A simple wideband Multiple Input Multiple Output (MIMO) antenna is demonstrated at millimeter-wave (mm-wave) frequency band. The antenna is intended for 5G applications in K band (18GHz-27GHz) and Ka-band (27GHz-40GHz) and 5G New Radio (NR) n257/n258/n261 bands. The proposed antenna is simulated using CST software. The antenna has four rectangular patches without any slot or extra structure positioned orthogonally on top of the dielectric material. In the bottom of the dielectric material, radiating elements have a connected ground structure with a circular ring and microstrip transmission line of a perfect impedance match for feeding the antenna. The connected ground is essential for improving isolation. The performance characteristics of an antenna are analyzed with reference to return loss, bandwidth, isolation, radiation efficiency, gain, current distribution and radiation pattern. The antenna successfully achieves the MIMO diversity performance during the simulation tests, making it a promising contender for fifth-generation mm-wave applications.