I've tried to research this and nearly all of the articles I found indicate that players that have been invited to a realm can still access it, even if the realm creator is not online.

However, this is not the case. 

My friend created a realm and I was able to access it and we were able to build within the realm together at the same time.  However, I cannot access the realm if he is not online.

Also, when we are both in the realm, if he leaves the game, I get kicked out.

We are both using Nintendo Switch to play.

If this is not a bug, please advise on how I can access the realm without him there.