Check that you've tried BOTH channel 12 and channel 37. (Quest was originally on Ch 37 but moved to Ch12. However it's gone back to Ch 37 on my Freeview TV after a recent retune).
Different devices have different 'sensitivities' (i.e. their ability to 'lock onto' weak signals) and different 'selectivities' (i.e. their ability to reject interference on neighbouring frequencies). Your TV has got a better level of sensitivity (and/or a better level of selectivity) than your Humax box has, which explains why your TV can 'pull in' the Quest signal but your Humax box can't.
I stand by my suggestion of installing a signal booster.