“Adjoining the territory
From the east side
To the west side,
Once again, in a very summary fashion of east to west, the territory of Reuben was next to Ephraim. However, in Joshua, chapter 13, Reuben was an east bank territory and had no land on the west side of the Jordan River. Somehow it would be south of Ephraim, maybe in the old Dan territory or the Benjamin territory. From this description, it is hard to tell.
“Moses gave an inheritance to the tribe of the Reubenites according to their clans. Their territory was from Aroer, which is on the edge of the Wadi valley of the Arnon, and the town that is in the middle of the valley, and the entire tableland by Medeba. They got Heshbon, and all its towns that are in the tableland. That includes Dibon, Bamoth-baal, Beth-baal-meon, Jahaz, Kedemoth, Mephaath, Kiriathaim, Sibmah, and Zereth-shahar on the hill of the valley. Beth-peor, the slopes of Pisgah, and Beth-jeshimoth, that is all the towns of the tableland, and all the kingdom of King Sihon of the Amorites, who reigned in Heshbon. Moses defeated the leaders of Midian, Evi, Rekem, Zur, Hur and Reba, as the princes of Sihon, who lived in that land. Along with the rest of those they put to the sword Balaam son of Beor, who practiced divination. The border of the Reubenites was the Jordan and its banks. This was the inheritance of the Reubenites, according to their families with their towns and villages.”
The tribe of Reuben got the most southern smallest part of this captured territory, around the kingdom of Heshbon, just north of Moab with the River Jordan as the western border. They got the tableland of Medeba with all the towns included. They also got Heshbon and the towns close by, about half of the kingdom of King Sihon.