A scientist and academic teacher who has been educating military and civilian students at the Faculty of Electronics of the Military University of Technology (WAT) for over 20 years. A specialist in telecommunications and reconnaissance and radio-electronic warfare systems, particularly in the area of the physical layer of radio communication systems. Currently, his scientific and research work focuses on the use of 5G technology in the military, military applications of drones and anti-drone systems, measurements and modelling of radio channels in the millimetre wave range. He is the head of the 15-member Radio Channel Analysis, Modelling and Estimation Team (ZAMEK-R), which is implementing four national and international research and development projects.

From 2017 to 2024, he served as a principal voting member of the Information Systems Technology Panel within the NATO Science and Technology Organisation. Since 2019, he has been a non-governmental expert on the EDA CapTech (Capability Technology Areas) Communication Information Systems and Networks panel at the European Defence Agency. From 2020 to 2024, he was involved in the work of the IST-187-RTG working group on 5G technologies application to NATO operations, and since 2023 he has been active in the IST-SET-204-RTG group on RF-based detection and classification of UAS. In 2021–2024, Colonel Kelner was appointed by the Minister of National Defence to the Minister of National Defence's Social Advisory Team for Cybersecurity. From 2022 to 2025, he served as an expert at the Office of Electronic Communications.

In 2001, he graduated with honours from the Military University of Technology (WAT) with a degree in technical physics. He defended his doctoral thesis in 2011 at the Faculty of Electronics of WAT in the field of telecommunications, for which he received the WAT Rector's Award and the title of Innovator of Mazovia. He obtained his postdoctoral degree in 2020 at the AGH University of Science and Technology in the field of technical informatics and telecommunications. In 2024, he completed postgraduate MBA studies in cybersecurity management at the Faculty of Cybernetics at the Military University of Technology. He also completed postgraduate studies at the Warsaw School of Economics, the Bydgoszcz Academy and the Military University of Technology.

From 2021 to 2024, he served as Director of the Institute of Communication Systems at the Faculty of Electronics at the Military University of Technology, and since September 2024, he has been Dean of the Faculty of Electronics at the Military University of Technology.