In this paper, a modified rhombus-shaped antenna for a sub-6GHz range of 5G is designed and analyzed using characteristic mode analysis (CMA). The proposed antenna is extended to form a 2×1 MIMO antenna with a wide bandwidth of 2600MHz (3.2-5.8GHz). The high level of mutual coupling (|S21|=-12.31dB at 4.75GHz) exhibited by MIMO antenna is mitigated with the help of modified hexagonal-shaped mu-negative (MNG) metamaterial unit cells. Using CMA, MNG unit cells are placed such that orthogonal modes are generated within MIMO antenna elements resulting in a high isolation level (S21>-25dB) for the entire bandwidth (with peak value of -34.4dB at 4.75GHz). This also aids in the miniaturization of MIMO antenna by the close placement of antenna elements with an edge-to-edge gap of 0.13λ (λ is the resonating frequency). The proposed MIMO antenna with MNG unit cells (called MNG-MIMO) retains wide-bandwidth (2600 MHz), a low envelope correlation coefficient (ECC <0.001), high diversity gain (DG=9.9dB), the high radiation efficiency of 82%, and exhibits circular polarization (Axial ratio <3dB) in the frequency range of 4.56-4.89GHz. In order to highlight the practicality of the proposed MNG-MIMO antenna, it is extended to form an eight-element MNG-MIMO antenna (called eight-element MNG-MIMO) with a large ground plane of 170×80mm2. The simulated response of eight-element MNG-MIMO indicates no change in efficiency and ECC levels with a minor variation in isolation is observed. Thus with a compact volume of 990mm3, the proposed design is fit for various 5G and IoT applications.