I recently went through the process of linking my GitHub account on the UE4 website. It successfully linked - I got a confirmation email from Epic Games, and my GitHub user settings page says that Epic Games is connected. However I am still unable to access the UE4 repository on GitHub (Yes, I am signed in.)
Could try to clear cache, another browser, it could be some login credentials left at the cache level keeping you down. If those solutions don’t work, your personal github page should have a link to the repo from you user page unger organizations, and you will see the epic logo. (you can find organizations on you profile page under your profile image, if you go through that link you might get a message about the admin inviting you on [DATE])
You can also get an binary version of unreal through Nvidia’s unreal fork on their github, its a few version behind usually but it has some extra physx stuff. here