The paper presents measurements of a Quantum Key Distribution system operating on a cross-border QKD link between Ostrava (CZ) and Cieszyn (PL). The system is part of the Horizon OpenQKD project. The study attempted to determine the maximum attenuation where the QKD system was still functional and also any correlation between the crucial parameters of the quantum bit error rate (QBER), secret key rate (SKR) and link attenuation (dB). Providing a statistical analysis of the measured data, the paper expands our understanding of the behaviour of the QKD link as it approaches its technological limits.