According to the specs on your computer
https://www.asus.com/us/ROG-Republic-Of-Gamers/ROG-GL502VM/specifications/, the speed of the 8mb of the installed RAM is 2133Mhz. Further, to run in dual channel, you need to have the same amount of matched RAM in both channels and you only have one empty slot for additional RAM (just becasue the RAM is dual ranked, does not mean it is running in dual channel). Most likely, the BIOS has a default increased clocking of the installed 2133Mhz RAM to 2400Mhz, however, when you added additional RAM, even though it is 2400Mhz RAM, the BIOS has reverted the RAM to the base memory speed of your system. I don't know if matching the installed RAM will clock it up the RAM or not, and I would run a system analysis, ie HWinfo64 to see if your system is running in dual channel.