Hi, even if I add to the walled gardens of the hotspot:
- www.youtube.com
- https://www.youtube.com
- m.youtube.com
- https://m.youtube.com
- ytimg.com (the player)

i cannot see youtube site but the rb redirects to the captive portal.

Any tips?

Thanks in advance, Simone.

There are more domains for youtube videos, at least googlevideo.com to add.
As it might depend upon the region, you are located in, you should use firefox+firebug to see, which domains you have to put into walled garden.