This study presents a coplanar waveguide (CPW)-fed octagonal split ring antenna (OSRA) that is augmented with a split ring resonator (SRR) for 5G sub-6 GHz applications, effectively mitigating in-band interference between n77/78 (3.30–4.20 GHz), and n79 (4.40–5.0 GHz). A solitary SRR effectively creates a notch band from 4.20 GHz to 4.40 GHz in an antenna that is downsized by 82.58% (20 mm × 20 mm) compared to a circular patch antenna intended for 3.50 GHz. The n77/78 and n79 bands are effectively delineated by the introduced notch bands, which assist in reducing interferences among in-band applications. The impedance matching and tuning of lower frequencies are modified by adjusting the CPW gap, ground length, and breadth of the extended ground, respectively. The notch band for interference reduction in in-band applications is achieved by relocating the position of the SRR along the antenna's length. The dual-band, cost-effective antenna attains gain values of 5.59 dBi and 4.16 dBi at resonance frequencies of 3.50 GHz and 5.06 GHz, respectively, with impedance bandwidths of (3.26–4.22 GHz) and (4.41–5.50 GHz). Consequently, the proposed antenna is highly suitable for 5G NR bands n77 (3.30–3.80 GHz), n78 (3.30–4.20 GHz), n79 (4.40–5.0 GHz), and the new Wi-Fi 5 bands: (5.17–5.19 GHz), (5.19–5.21 GHz), (5.21–5.23 GHz), (5.23–5.25 GHz), (5.25–5.27 GHz), (5.29–5.31 GHz), (5.31–5.33 GHz), (5.35–5.37 GHz), (5.37–5.39 GHz), (5.39–5.41 GHz), (5.41–5.43 GHz), (5.43–5.45 GHz), (5.45–5.47 GHz), (5.47–5.49 GHz), (5.49–5.51 GHz). The measurements of the antenna results obtained from the Microwave Analyzer N9916A and the anechoic chamber are in great concordance with the simulated antenna results.