It doesn't work

The image with the portal has a lot less noise. I can't see in the screenshot if you surely turned the portal setting for the light on though. Keep in mind it's not a magic bullet, the portal helps out with the noise, it doesn't get rid of it in its entirety. What is black magic though is the denoiser, that stuff is math magic.

Like Omar said,

It is easier to see if you compare them  together, at the framed areas, it might be more noticeable:

Enabling Path Guiding (CPU only, at the moment) also helps clearing Noise faster aa well as Denoising...and you can of course combine them.