Frequency-selective surfaces (FSS) serve as openspace filters to block certain radio transmissions in controlled environments, with applications such as enhancing cybersecurity. This paper presents a novel unit cell design for a two-dimensional FSS, consisting of four half-moon elements, specifically optimized for 5G FR1 (Frequency Range 1) bands. We propose a single-layer FSS unit cell capable of dual-band frequency response. Full-wave electromagnetic simulations are conducted to evaluate the performance of the proposed structure, and parametric analyses are performed to derive design criteria that model the influence of key parameters on FSS behaviour. The resulting equations enable tuning of the design for other frequency bands and application-specific requirements, demonstrating the versatility of the proposed approach.