A young Monstera adansonii or Monstera deliciosa almost always starts with solid, unsplit leaves. That's normal, not a sign of a sick plant.
Split leaves — fenestration — are a response to two things: maturity and light. A cutting has to grow past its juvenile phase before it can produce fenestrated growth at all, and no amount of fertilizer speeds that up. Most indoor monsteras need several new leaves, sometimes a year or more of growth, before splitting begins.
Light is the lever you actually control. In the wild, monsteras climb toward brighter gaps in the canopy, and splitting is thought to help light reach lower leaves and let wind pass through without tearing the plant apart. Indoors, a monstera kept in low or medium light will often keep producing solid leaves indefinitely, no matter how old it is. Move it somewhere with bright, indirect light — close to an east or west window, or a few feet back from a south-facing one — and new growth usually starts showing splits within a few leaves.
A support pole helps too. Monsteras are vining araceae; a plant allowed to climb tends to mature and fenestrate faster than one left to sprawl.
What won't help: more water, more fertilizer, or repotting. Overwatering in particular just risks root rot while you wait.
If a plant is getting strong indirect light, has something to climb, and is still putting out entirely solid leaves after a year, patience is still the right answer before intervention.