Legacy field protocols such as Fieldbus and HART provide deterministic communication for traditional process control; however, their limited bandwidth, scalability, and adaptability constrain reliable communication in large-scale, data-intensive Industrial IoT (IIoT) systems, particularly under high node density, mobility, and interference conditions. This paper presents a Hybrid Decode–Amplify–Forward (HDAF) protocol within a Fog-assisted cooperative relay architecture to address these limitations. The proposed HDAF scheme adaptively switches between Decode-and-forward (DF) and Amplify-and-Forward (AF) modes under Nakagami-m fading, leveraging cooperative Fog relay nodes to enable low-latency, and interference-resilient communication. The framework integrates low-power IEEE 802.15.4 (Zigbee) sensor networks with a 5G New Radio (NR) back haul via dual-radio IoT–Fog gateways, facilitating traffic aggregation and Quality-of-Service (QoS) mapping. The software-defined coupling mechanism further supports real-time analytics and adaptive relay operation. Validation using theoretical analysis, MATLAB simulations, and field experiments conducted on a gas-processing-plant-inspired industrial testbed demonstrates up to a 95.8% reduction in Bit Error Rate (BER) and approximately 96% improvement in channel capacity compared with non-relay schemes. These results confirm that the proposed IoT–Fog HDAF protocol effectively extends coverage, mitigates interference, and provides a scalable and cost-effective solution for high-reliability, low-latency IIoT communications in dense industrial and5G-enabled environments.