I don't understand why this is happening. Some sites work, some of them stopped working. Moreover, these are necessary and good sites with useful information.
It seems to me that the problem is in the provider. Try to use any vpn service.
I have the same problem. Moreover, many sites that didn't open were needed for me to prepare for exams, to write graduate work. In the end, I just found a proven vpn service https://fastvpn.com/ that I've been using for some time. It is very convenient, because this vpn service doesn't interfere with me at all when I'm surfing the Internet.
you should watch this video to fix the problem http://adfoc.us/73602182914397