It just doesn't work
https://imgur.com/tpNcVmM - without portal
https://imgur.com/ZWO7ZW4 - with portal
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.