Cannot play multuplayer with my son when on different wifi

I've raised this issue several months ago with xbox live/microsoft, but they can solve it.

My son and i can play multiplayer minecraft if on same wifi, but not when on different wifi (when I'm travelling)

He is a child account attached to mine

If I create another adult account, we can play, but not with his account Noah72016.
My account is puzzigully.

The microsoft people have ALL the testing and data