Here's how to access GitHub despite a DNS outage. This tutorial is for Mac, but the general idea applies to other operating systems, too.
1) Adding GitHub to your hosts file
First, open your hosts file with the following command:
sudo nano /etc/hosts
You don't have to use nano; I'm just using it for this walkthrough. Now add the following lines, with tabs in between each IP and the domain name:
18.104.22.168 github.com 22.214.171.124 avatars3.githubusercontent.com 126.96.36.199 assets-cdn.github.com
Save with Ctrl+O, and exit with Ctrl+X.
2) Flushing your DNS
This part varies based on the version of macOS you're running. I'd just follow this tutorial: https://www.igeeksblog.com/how-to-flush-dns-in-mac-os-x/
Now you should be done. Let me know if you run into any issues.