How can I avoid cakes to turn into jelly?
It could either be a problem of heat transfer or leavening.

Heat Transfer

Probably the bottom "jelly" potion of the cake is not getting enough heat. I can imagine some possibile causes. 


Ovens that are not fan assisted are generally warmer at the top than at the bottom. Generally you should have the cake in the middle of the oven, but for some ovens you may need to move the cake higher.
Plastic bakeware (or silicone if that's what you have) typically conducts heat less efficiently than glass. And glass has poorer heat transfer than metal, so try using a metal cake form.
The oven temperature might be wrong; oven dials are often wrong by a few degrees (I once had an oven that was out by ten degrees C). Get an oven thermometer and put it on the same shelf as the cake.


Another possibility is that you just need to leave the cake in longer, but from what you say it sounds like the upper portion of the cake would then be overcooked and dry.

Leavening

In your ingredient list you didn't mention a leavening agent. That could be a problem in itself if you are expecting air bubbles. Typically you would use baking soda or baking powder, or a combination of those two (depending on the acidity of the batter) to introduce air.

It is possible to introduce air without a chemical leavening agent by, for example, folding whipped egg whites carefully into the batter.

In either case, air does not "flow into the cake". It needs to be introduced either before baking or by a leavening agent during baking.