The oven is impossible to get clean with ANY brand of oven cleaner. The instructions for cleaning in the instruction manual supplied suggest you 'leave a rag soaked in ammonia inside the closed oven for several hours'. Not only is this totally ineffective it is also dangerous. After the ammonia soaked rag has been left in the oven for several hours, the fumes when the oven door is opened again irritate eyes, respiritory system and skin despite wearing protective gloves, goggles and mask.
It is possible to buy a better cooker for half the price.