Best thing to do is find a magnet to suck it out. Because there are many sundries in the sewer, it is not good to hook out with things like iron hooks. But the method of using a magnet is very good, no matter what the environment in the sewer, you can not see it. Many years ago, my key was once lost in the sewer, and I was so anxious that I found a magnet to suck it out directly. I hope it will be useful to you.
The key into the toilet sewer can be hooked out with a wire, because the key is a key ring, plus the key has weight, and the sewer also has a bend, it is impossible to rush down, so long as you take a meter long than the sewer bend a little longer, and then twist a hook with pliers at one end, and use a flashlight to hook out the key.
With a magnet attached to the front of the rope (make sure it's firmly attached) flush the magnet down the drain with water, try sliding it up and down, left and right, and slowly lift it up, see if you can pull the key up.
The key falls down the toilet drain and pulls it out with a magnet.