Telkom Mobile Data Restricts certain websites

github.com and stackoverflow doesn't load. Every other website loads. If I change the sim in my router for any other ISP, e.g. vodacom, it loads fine.

After much hassles back and forth they told me to use a APN called 'unrestricted', which gives me github.com access, but not stackoverflow.com

When I use Telkom mobile data I get the errors in loading websites.(when I connect via any other providers I dont). When contacting Telkom for help, they said the problem is on stackoverflow.com's side. And then hung up on me. Well I won't be renewing my contract.