Indeed, I didn’t put anything in CSS, now I see where was the problem. Following the Google Font official guide for using Noto fonts on the web that you mentioned, I modified the font of my site to Noto Sans SC with success! 
Thank you!!
For those who may have run into the same issue as mine (failed to set up Noto non-European language fonts on my site through Google Font API), take a look at the official guide mentioned above.
And a tip for those who wish to use Noto Sans CJK, remember to put Noto Sans before Noto Sans TC (or SC , JP etc. in the font-family property to ensure a good look for alphabetic characters.
Example:
body {
font-family: 'Noto Sans', 'Noto Sans SC', sans-serif;
}