Future wireless networks demand capabilities beyond traditional communication, driving the development of Integrated Sensing and Communication (ISAC) for environmental awareness, localization, and tracking. Ubiquitous cellular deployment allows ISAC to maximize spectral efficiency, lower costs, and expand sensing coverage. However, sub-6 GHz research has heavily favored communication, leaving sensing capabilities largely underexplored. To bridge this gap, we introduce CellSense, a novel sub-6 GHz ISAC architecture natively integrated into the 5G cellular protocol stack for real-world target tracking. We validate the system via Sionna-based orthogonal frequency-division multiplexing (OFDM) link-level simulations and an experimental USRP hardware prototype using the OpenAirInterface (OAI) stack. Furthermore, we analyze the communication-sensing tradeoff by quantifying how pilot symbol density impacts throughput versus sensing accuracy. Simulations show that CellSense achieves a 74 percent detection probability with a 1.43 m localization error in indoor warehouse environment, which improves to 94 percent detection and a sub-meter error of 0.33 m in the outdoor environment of Oval area at the NCSU Centennial campus. Hardware experiments in a highly cluttered indoor laboratory confirm a 1.28 m localization accuracy and 76 percent detection probability, proving its efficacy for practical ISAC deployments.