We present results from quasi-simultaneous, broadband (1 through 50 GHz), full-polarization, multi-epoch JVLA observations of the fast-rotating ultracool dwarf (UCD) LSR J1835+3259 (=LSRJ; P_rot = 2.84 hr), aimed at clarifying the dominant mechanisms behind its radio emission. We find that circularly polarized emission dominates over 30% of LSRJ’s total rotation phase, i.e., 0.84 hr. The circular polarization is confined to specific rotational phases and is absent during the quiescent intervals overall. During the remaining ~2.0 hr of the rotation period, the radio emission is dominated by quiescent, non-circularly polarized emission.A wideband dynamic radio spectrum reveals a range of localized bursts and frequency drifts across multiple bands. We successfully model the circularly polarized emission as auroral radio emission originating from a single active field line, where electrons with energies around 2 keV radiate via the electron-cyclotron mechanism. The model estimates both the emission height along a dipolar magnetic field and the cone opening angle implied by cyclotron-maser emission physics, as a function of the observed frequency.The quiescent emission is well described by a self-absorbed synchrotron power law with α = -0.62 ± 0.09. This emission arises from a radiation belt around LSRJ, fed by a population of relativistic electrons with index p = 2.24, reaching energies up to about 20 MeV. Our observations and modeling provide a consistent picture for LSRJ: 2 keV electrons drive the auroral emission, while ultra-relativistic electrons with energies up to 20 MeV supply the radiation belt. With temperatures similar to those of giant exoplanets, UCDs also share similar atmospheric chemistry and magnetic phenomena. As one of the closest and brightest ultracool dwarfs, LSRJ serves as a key laboratory for studying such processes. Our findings therefore significantly advance our understanding of magnetism in giant exoplanet–like atmospheres.