Sealing the Servants of God
In order to examine the text in it's proper context we must first answer the question: Is it Literal(Literally describing what is understood in it's Historical/Grammatical Context), or Figurative (Representative of Another Reality)?
We have the names of the Historical Tribes of Israel mentioned, but we have 2 Glaring Omissions, the Tribe of Ephraim, AND The Tribe of Dan. The OP's question never refers to the Tribe of Ephraim being excluded, although Ephraim is not mentioned by name because.....
The Tribe of Joseph is mentioned!
The Tribe of Manassas(Manassah) IS Mentioned, leaving the reader with the distinct impression that BOTH TRIBES of Joseph are included, yet for some unknown reason from the text, the Tribe of Dan is missing.
The Importance of "12"
To correctly understand this passage, one must "follow John" up to Heaven where he sees Angels, who have numerous assignments: They 'stand' on the 4 corners of the Earth, 'holding' the winds (of Judgment) back, they are also charged with Sealing the Servants of God in their foreheads. Since this action of "Sealing" is critically important to understanding this passage, it must be understood what sealing means.
"σφραγῖδα"(sphragida) is the word "seal" used in Rev. 7:2 and it can mean "a signet-ring", or "that by which anything is confirmed, proved, authenticated, as by a seal (a token or proof)", (from Thayers Lexicon.) Since "What the Seal Looks Like" isn't described in the passage, but the Tribes of Israel are "authenticated as being sealed" are, we must conclude that this passage is meant Figuratively, as the actual physical expression of the seal is absent, but the understanding of being "Marked, set apart and authenticated" is understood.
This takes us to the next understanding we must have, which is critical to answering the OP's question, and that is the significance of "12". There are "12" times 1000 sealed in each tribe of Israel, and the combined number is mentioned in vs. 4-"144,000".
"12" from many sources Figuratively means the "Government of God". There are 12 Tribes of Israel, 12 Apostles, 24 Elders(12x2), and it's no accident that "12" and the multiple of 12 is mentioned as "God's elect". Furthermore, there are 12 Gates in the enterance of the New Jerusalem(Rev. 21:12-13), 12 Foundations(vs 14), 12 Pearls on the 12 Gates; the importance of the number "12" cannot be overstated.
"The Lord is There"
In the Book of Ezekiel, the last 8 Chapters are described as Ezekiel's Future Visions of the Temple and the Tribes of Israel. In Chapter 48, waters issue forth from the House of the Lord into a river which cannot be passed over(Ezek. 47:4). What is significant is that these waters are intended for the healing of the nations, which parallel Rev. 22:1. Since there has been no noticeable river issuing from the Temple from Ezekiel's time onward, we must conclude that this is a Future Prophecy, waiting to be fulfilled. But what is significant is in Ezek. 48:32 (KJV),
"And at the east side four thousand and five hundred: and three gates; and one gate of Joseph, one gate of Benjamin, one gate of Dan".
This parallels Rev. 21:12 (KJV)
"And had a wall great and high, and had twelve gates, and at the gates twelve angels, and names written thereon, which are the names of the twelve tribes of the children of Israel."
At the end of the Ezekiel passage (vs 35) it reads:
"and the name of the city from that day shall be, The LORD is there."
Reconciling the Tribe of Dan
It becomes important therefore, to reconcile the Tribe of Dan, not mentioned in Rev. 7, yet mentioned in Ezekiel 48:32, and by inference, mentioned in Rev. 21:12. If we read Rom. 11:26 (KJV), "All Israel shall be saved", we don't read, 'All Israel shall be saved EXCEPT THE TRIBE OF DAN'. The argument of Dan's pursuing of Idolatry in Judges 18 is offset by ALL the tribes of Israel pursuing idolatry at some point of their history-the Tribe of Benjamin in the following chapters of Judges were almost destroyed to a man, because they took sides with "Sons of Belial" who defiled the Levite's concubine, after attempting to defile the Levite himself.(Judges 19).
The answer to resolving the absence of the Tribe of Dan in Rev. 7 goes back to the Figurative understanding of the number "12". Since "13" ACTUAL TRIBES occupied the land God gave the Children of Israel; Joseph being given 2 portions, Ephraim and Manassah, then how do you make "13" equal "12", the number of wholeness or completion in the Government of God?
The Answer is beautiful: mention Joseph, but don't mention either Ephraim or Dan. Since we are understanding "Figuratively", we know Joseph contains BOTH Ephraim and Manassah, yet by mentioning "Manassah", we know BOTH sons of Joseph are included. But Joseph didn't have a portion in the land, his sons did, so Dan, the Other Unmentioned Tribe, is in fact "Included" in the 144,000 as this number is Symbolic and not an actual tally of individuals. The Gates of the New Jerusalem mention Joseph and Dan; but the numbers of the individual pertaining to those gates would be "12,000" for Dan, and "24,000" for Joseph-consequently they aren't mentioned as such. Ephraim and Dan are being excluded, yet Joseph and Manassah included arrive at the Perfect Number of 144,000!
The Tribe of Dan(and Ephraim) being excluded from Rev. 7 resolves a Figurative understanding of the number 12; it's God's way of making a "Baker's Dozen" equal the perfect number of 12.