How to be an Xbox Insider Alpha Skip Ahead?

You need to either be in the uk or us and be an xbox insider on ring beta, alpha or alpha skip ahead.

You must be invited to become an Alpha Insider or Alpha Skip Ahead Insider, to do so You must Very active in Xbox, doing missions and report the bugs You see in the system, as much as You do this, so You Will be Very close to become an Alpha Insider. You
must be at least in Delta preview to become an Alpha Insider, but You Just need to be an Insider to be invited to become an Alpha Skip Ahead, but this one is very rare to be invited and there is a certain number of people to be invited, no more than that,
If You were not invited to be Alpha Skip Ahead certain time, so You must wait a Second chance when they Will invite New members.

Only xbox insider alpha skip ahead users are getting the experimental dashboards.

You tryed ask for support in http://beta.suport.xbox.com/contactus ? maybe there if you chose Hub and then talk with some support agent maybe he/she can help you

"xbox not being able to play dolby atmos"

According to the Latest Alpha skip ahead Insider release notes this is still an issue the engineers are tracking and working on.