HELP: Basic Cue Issue DDJ-RX

hi there! New to DJ-ing. A friend sold me their DDJ-RX, and I've been having trouble cueing songs properly. I have "Master Cue" and "Cue" ON, but the tracks still keep going through the main speaker-- I have my headphones plugged in and I know they work. Running on a Mac. I've tried just about every sound setting you can think of and nothing seems to work-- In Sound Settings, under Output > Headphones I have it selected as "DDJ-RX Phones" but when I cue songs, they play straight to the main speaker and nothing comes through the headphones. I got a dual audio output to try having both output channels through my computer and that didn't work either-- the songs were just coming in through my headphones and the speaker (the music did fade when I hit cue but it was still coming out of the main speaker). I'm convinced I have to change the settings in Rekordbox or my computer because my friend did not have this problem. Or I'm just missing something completely basic. I really would appreciate any and all suggestions because cue-ing songs and beat matching before playing is the bulk of what I want to do.