Strictly speaking, Numbers 26:53 is not exactly correct.
To the above shall the land be allotted as territories, in proportion to the number of names.
The verse follows the census of the 12 tribes just enumerated in chapter 26 and foreshadows the allotment of the land. In fact, the verse is inaccurate on 2 counts: but the issue with the latter half of this verse and the next, I will post separately.
The fact of the matter is that “the land”—always understood as the promised land of Canaan (see the borders of the land as defined in Num 34:1-12)—is not apportioned following the 12-tribe census just enumerated. For as we know from other traditions preserved in and outside of the Torah—namely Numbers 32, Deuteronomy 3, and Joshua 13—2½ of the original 12 tribes elected to take portions outside of the promised land, in Transjordan.
Specifically, Num 26:53, which claims that the land is apportioned according to the list of 12 tribes previously enumerated, contradicts Num 34:13 and Joshua 13:7, each of which specifically articulates that the land is to be apportioned according to 9½ tribes.
“This is the land that you shall give as legacies by lot, which Yahweh commanded to give to the 9½ tribes.” (Num 34:13)
In other words these verses and their immediate contexts acknowledge the Transjordanian territories of the Reubenites, Gadites and half tribe of Manasseh. The author of Numbers 26:53 seems to have forgotten it (not that serious of an error) . . . or, he was unfamiliar with the Transjordanian tradition or was writing prior to its creation ! ?
It is not improbable to think that a group of Transjordanian Yahweh/El worshipers in the 10th or 9th centuries were accepted as Israelites and grafted onto the 12 tribes tradition (indeed scholars argue that this itself is a late creation) and even given an origins story that explained how they were “allotted” the land they currently posses. The Machirites (the half tribe of Manasseh living in Transjordan) might have been such a group. Remember, these origin stories were often written to legitimate the possession of contested land in later centuries, such as both the Yahwist tradition and the Moabite stela suggest, with divine decree, concerning the proper ownership of northern Moab (see #282-285 & #287).
Strictly speaking those tribes hadn’t requested permission to settle in Transjordan yet.
Speaking of the Mesha stele, it mentions the people of Gad in a way which implies they were there before the Israelites fortified the area. Gad is not mentioned in the Song of Deborah, which means they might have become part of the Israelite tribal confederation later than that. Mesha does not mention Reuben at all as far as we know.