The antenna designed in this study was a monopole double-layer rectangular patch antenna. The antenna’s bandwidth and radiation efficiency were improved by adding a frequency-selective surface (FSS). In the reference article, an FSS was positioned beneath the shortened length of the ground level surface to serve as a partially reflector for the antenna’s rear lobes. While many antennas have been developed without this reflector, the addition of the FSS proved beneficial. The antenna was constructed using a 48 × 48 × 1.6 mm3 lower-loss RT Duroid 5880 substrate for the microstrip rectangular patch antenna with an inset feed. The FSS, on the other hand, was manufactured on an identically sized FR4 substrate. The FSS consisted of an array of uniformly spaced array of 9 × 9 swastika-shaped unit cells. The front-to-back ratio (FBR) drastically improved, in addition to increased bandwidth, radiation characteristics, and gain, once the FSS was implemented. The FSS structure was located at a height of 6 mm. The proposed antenna was found suitable for various uses for 5G sub-6 GHz bands such as n46, n47, n77, and n79.