A cryogenic quantum dot thermometer is calibrated and operated using only a single nongalvanic gate connection. The thermometer is probed with radio-frequency reflectometry and calibrated by fitting a physical model to the phase of the reflected radio-frequency signal taken at temperatures across a small range. Thermometry of the source and drain reservoirs of the dot is then performed by fitting the calibrated physical model to new phase data. The thermometer can operate at the transition between thermally broadened and lifetime-broadened regimes and outside the temperatures used in calibration. Electron thermometry is performed at temperatures between 3.0K and 1.0K, in both a 1-K cryostat and a dilution refrigerator. In principle, the experimental setup enables fast electron-temperature readout with a sensitivity of 4.0±0.3mK/√Hz, at kelvin temperatures. The nongalvanic calibration process gives a readout of physical parameters, such as the quantum dot lever arm. The demodulator used for reflectometry readout is readily available at relatively low cost.