I'm training playing drums for about 3-4 months, but I still can't control my feet separately from my hands (i.e. when I play some rhythm with my hands I can barely play some other rhythm and keep it clear with my feet.)
What am I missing?
You are missing independent coordination between your limbs. Your brain has not yet developed neurological connections that supports such kinesthetic interdependence as it is something that takes time to do - some of us longer than others.
In order to develop strict and evenly developed competence with all of your limbs, it therefore stands to reason that you practice each / every possible limb combination with a given rhythm. For example:
I would recommend doing this without a metronome at first (and very, very slowly!) so that the focus is on learning the limb coordination. Once coordination is understood, a metronome may be introduced.
As always, I also recommend a private teacher is possible.
Granted, over time, the process suggested above will become less and less necessary, but understanding coordination is paramount.
Hope that helps.