In remote rural areas, cellular IoT coverage remains limited and exhibits geographical variations, often leading to unreliable connectivity or even complete lack thereof. Mobile operators cannot afford comprehensive coverage assessments conducted through drive testing to provide coverage maps, hindering a wider adoption of IoT solutions for precision agriculture. However, system integrators and farmers could conduct ondemand field coverage assessments to evaluate the feasibility of embracing cellular as a Low Power Wide Area Network (LPWAN) communication technology in their production units if an affordable tool were accessible. This work proposes an open-source tool for cellular IoT coverage analysis. The system comprises a low-cost, portable, and autonomous device with a Global Navigation Satellite System (GNSS) chip and a cellular modem. Constructed using affordable, interconnectable kits (Arduino Zero and a Quectel BG96 modem board) powered by a power bank, it offers several hours of energy autonomy. The device collects network information and transmits it in georeferenced form via MQTT (Message Queuing Telemetry Transport) to a server for storage and user-friendly visualization. The server side is composed of four open-source systems. Eclipse Mosquitto serves as the system's core, implementing the MQTT protocol. Telegraf, a server-based agent, subscribes to Mosquitto topics, extracts data from JSON-formatted messages, and sends it to InfluxDB, a time-series database, for storage. Finally, Grafana serves as a powerful web-based data visualization system, retrieving and displaying InfluxDB data through a color-coded map. Experimental results demonstrate the exceptional potential and effectiveness of the proposed solution.