This paper presents a compact dual frequency Substrate Integrated Coaxial Line (SICL) based Cavity Backed Slot Antenna (CBSA). A primary slot along with secondary slot is etched on the bottom plate of SICL cavity to generate two non-adjacent resonant frequencies. The primary slot is placed such that it is excited by {\text{TE}}_{110(even)}^x mode of the SICL cavity. The secondary slot is placed near the primary slot and excited by the SICL feed line. At second resonance, SICL cavity acts as the reflector for the back radiation and significantly improves front to back ratio (FTBR). The size of the proposed SICL cavity backed slot antenna is found to be compact in comparision with other technologies based design including Substrate Integrated Waveguide (SIW) cavity backed antennas proposed earlier. The antenna resonates at 24 GHz and 38 GHz with gain of 5.1 dBi and 4.8 dBi, exhibiting high FTBR of 14 dB and 28 dB with co to cross polarization ratio of -22 dB and -32 dB respectively. This antenna finds its application in millimetre wave band 5G spectrum coverage for customer premises equipments (CPE). © 2021 EurAAP.