A cylindrical single crystal SrLaAlO4 Whispering Gallery mode dielectric resonator was cooled to millikelvin temperature using a dilution refrigerator. By controlling a DC-magnetic field, impurity ions’ spins were coupled to a variety of modes allowing the measurement of hybrid spin-photon systems. This Electron Spin Resonance mapping technique allowed us to detect Cu2+,Fe3+ and Mn4+ impurity ions (at the level of parts per million (ppm) to parts per billion (ppb)), verified by the measurement of the spin parameters along with their site symmetry. Whispering Gallery modes exhibited Q-factors ⩾105 at a temperature less than 20mK, allowing sensitive spectroscopy with high precision. Measured hyperfine line constants of the Cu2+ ion shows different parallel g-factors, g‖Cu, of 2.526,2.375,2.246 and 2.142. The spin-orbit coupling constant of the Cu2+ ion was determined to be,λ≃-635cm-1. The low-spin state Fe3+ ion's measured parallel g-factor, g‖Fe, of 2.028 reveals tetragonal anisotropy. The Mn4+ ion is identified in the lattice, producing hyperfine structure with high-valued g-factors,g‖Mn, of 7.789,7.745,7.688,7.613,7.5304 and 7.446. The hyperfine structures of the Cu2+ and Mn4+ ions show broadening of about 79G between 9.072GHz and 10.631GHz, and 24.5G broadening between 9.072GHz and 14.871GHz, respectively.