Problem with wall mounted gasket sealing

My wall mounted toilet’s gasket was leaking. I purchased the spongy type of gasket. Now when I installed the gasket, I noticed that the waste pipe is only sticking out of the wall between 1/4 to 1/2 inches.

When I installed the toilet, I was concerned that the tiny amount of pipe sticking from the wall would not be able to press in upon the gasket to make the seal.

So anyway the gasket is leaking.

How can I increase the effectiveness of the seal??

I was thinking that I could apply plumpers putty to the well that the gasket sits within the toilet. This would force the gasket to stick further out of the toilet, allowing it to be sealed better by the lame pipe.