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:
Begin with the most simple rhythm / beat patter you are studying.
Break it down into individual motions (one motion per limb being utilized.)
Learn each motion to competence.
Next, pick one of the motions and add it to one of the other motions, working to keep both consistent.
Once proficiency is reached, repeat with all two-motion combinations.
One proficiency is reach, add a third motion.
Repeat until all motions for all limbs are learned and can be performed consistently.
Add them all together.
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.
I play Piano but it is a very similar thing with coordinating each hand differently.
The trick is not to think of each limb playing its own part but instead to think of it as all four limbs playing one part. Don't play the bass drum with your left foot and the snare with your right hand, play the drum kit with your body. Like typing on a computer keyboard. Both your hands are doing different parts of the same task. Once you nail this aspect of it and get your head around how to approach it then I guarantee you will question whether or not you did actually play that.
I get chaos when I try to assign each limb a separate task. I will have to -when I get this really practiced, concentrate on the whole, but make the kickdrum foot not just take a turn in the beat, but drive it all, and the muscle memory almost has to have a mind all its own, or entirely, rather, and be thinking enough beats ahead to throw style in there.
I refer you to Gene Peterson's explanation of the way he developed his impressive independence, in my contribution on a similar question:
My advice (drummer of nearly 30 years) is not to think of your limbs doing things separately but as either doing things together or individually.
For example a simple beat with 8's on the hihat, beats 1 and 3 on the kick and 2 and four on the snare can be thought of as...
1 - hihat + kick & - hihat 2 - hihat and snare & - hihat 3 - hihat + kick & - hihat 4 - hihat and snare & - hihat
Once you get passed this, you can start adding notes and thinking about the timing between notes (triplets etc).