Hm.. need more information if possible! What window and how much sunlight is he getting? Did you check the soil for pests? If not, you should! Take a little stick if you're too nervous to use your fingers and move the top inch of the soil (carefully as to not damage anything) a bit and pay close attention to any tiny thing moving, could be fungal gnats! If not then it sounds like it could be root rott? because 3-4 cm doesn't sound like it's enough for it to dry out, in my experience. I usually wait until the top 1-2" is dry, usually 2" because our monsters like to be fairly dry before watering again. Are you humidifying? Not a big deal if you're not (Monstera's are pretty hardy without it) but it may help!

Thank you for this! No bugs or anything around the plant. I replanted him 2 weeks ago when I got him and his roots were fine. For those 2 weeks, I watered him only twice. He is in a plant bag as opposed to a plastic pot to allow for better breathing. He is in a mix of orchid bark, perlite and soil for green plants. I am humidifying, the humidity around him is about 60% but now I was adviced to raise it to 80% so thats what Im doing. He is in the bathroom (when buying plants I make sure that they are in a bit of a quarantine to not infect my other plants haha) where he is getting non direct light from above if it makes sense and Ive turned on a growing light for him. I dont know what to do with the plant... :( He did grow a new leaf for the time he has been here but it appears to be smaller than expected (or at least smaller than what the seller told me it would be as he told me every new leaf should be bigger than the previous ones).

Hello, Hu. :) Transplant shock is made worse by the low light. You can't always expect to keep all the leaves after transplanting. The plant will give up on some of the lower leaves, causing them to die like that, since it can't support that many leaves in the low light, especially when stressed. You've gotten some questionable advice I think about ideal conditions for these, or instead let's say you got correct info without needed context: It's true that growing them in a greenhouse under very high humidity and super-free-draining soil where water just easily pours through is close to ideal. However neither that type of soil nor the high humidity is necessary for an amazing successful plant; and focusing too much on those two factors has caused you to miss the most important limiting factors for your plant: more light & more water.

People all over the world grow these to amazing huge leaves in a plain dry house with plain cheap soil or even garden dirt, even though neither is "ideal". I'm afraid your search for the humidity of the bathroom has led you to far-too-low light conditions; and that beautiful greenhouse-style "perfect" aroid soil also requires perfect watering. By being so free-draining, It can cut your margin of error on when you must water in half.

So make sure you are getting water to it often enough, and perhaps consider moving it to a higher light spot. I'm not sure what kind of grow light you got for it, but those can often fall short because they focus too much on the spectrum (marketing) instead of the needed intensity.

Btw, your seller's comment about a bigger leaf every time was correct for that plant if you didn't repot it and kept in similar light and watering conditions, but transplant shock often sets back the size of new leaves, especially combined with lower light.