I just tried to watch a D tube and it worked fine for me.

Sometimes DNS resolution will give a 404 if your ISP hasn't configured it properly (or if they've messed with it and F'd it up) If you try google, open DNS or cloudflare DNS on your laptop that should eliminate that as the issue if it still doesn't work.
https://www.howtogeek.com/164981/how-to-switch-to-opendns-or-google-dns-to-speed-up-web-browsing/