one of the best answers to the question of why
Dan and Ephraim are missing in Rev 7:4-8, is presented in
the following video:
In short, the author of the video refers to Deuteronomy
29:18-21, which reads that the man, women, family of tribe
that introduces idolatry into Israel should have its
name "blotted out" from under heaven, and be separated out
of the tribes of Israel.
Interestingly enough, the Tribes Dan and Ephraim were
guilty of when they permitted Jerobeam to set up the
"Golden Calves" to be worshiped, one at Dan in the "Tribe
of Dan", and the other at Bethel in the "Tribe of Ephraim"
(1. Kings 12:25-30).
Keep looking up!
Maranatha! Josua