What car is it?
Without knowing anything about your vehicle, I'd guess the transmission drain plug likely does not allow you access to the full volume of oil inside, only a fraction of it.
In this case, without disassembly, the only solution is draining out the oil, measuring the quantity, adding the correct quantity and type of oil, drive a few hundred kilometers, repeat. Repeat this as frequently as you can afford, praying it improves.
Do this a few times and you'll have changed out the majority of the contaminated oil.
Remember: even 4 CVT oil changes are cheaper than a scrapyard transmission replacement.
