Fresh Windows 10 install won't switch to Insider Preview build

I updated my Windows 8.1 to Windows 10. I wanted to try out the Windows subsystem for Linux, so I went in to Settings -> Update -> Advanced options and selected Preview build with the fast ring option. However, I when I run Windows update, it doesn't get the preview build. I read that a Windows 10 install won't let you update to the preview build within 30 days of the install. Is this true? Is there a way around it? I installed Windows 10 all over again and it gave me a waing that I won't be able to recover my Windows 8 and I went ahead. However, it still doesn't show any Insider Preview builds when updating.