I'm not sure that's possible on the Comcast gateway. You can block a domain completely, but it's blocked for everyone. If you're only looking to enable YouTube Restricted Mode for certain devices, you can turn on Parental Controls from Xfinity xFi.
I don’t really understand what you mean because it works for everything I’ve tried so far except YouTube. YouTube is the only thing I’m unable to block on only certain devices. And xFi doesn’t allow you to block anything specific. That ends up being another all or nothing choice for blocking the devices entire internet use or not.
Is it possible you have the X1 system which includes the Youtube app? X1 parental controls might block it.
I am having the same issue. I setup parental controls to block YouTube, but Youtube is blocked from one Mac, but not another (confirmed not set as "Trusted"). Did you find any fix for this issue?