Pisgah and Mount Nebo are located in what had been allotted to the tribe of Reuben:
16 And to the Reubenites and the Gadites I gave from Gilead as far as the River Arnon, the middle of the river as the border, as far as the River Jabbok, the border of the people of Ammon; 17 the plain also, with the Jordan as the border, from Chinnereth as far as the east side of the Sea of the Arabah (the Salt Sea), below the slopes of Pisgah.
-Deuteronomy 3:16-17 (NKJV)
15 And Moses had given to the tribe of the children of Reuben an inheritance according to their families. 16 Their territory was from Aroer, which is on the bank of the River Arnon, and the city that is in the midst of the ravine, and all the plain by Medeba; 17 Heshbon and all its cities that are in the plain: Dibon, Bamoth Baal, Beth Baal Meon, 18 Jahaza, Kedemoth, Mephaath, 19 Kirjathaim, Sibmah, Zereth Shahar on the mountain of the valley, 20 Beth Peor, the slopes of Pisgah, and Beth Jeshimoth— 21 all the cities of the plain and all the kingdom of Sihon king of the Amorites, who reigned in Heshbon, whom Moses had struck with the princes of Midian: Evi, Rekem, Zur, Hur, and Reba, who were princes of Sihon dwelling in the country. 22 The children of Israel also killed with the sword Balaam the son of Beor, the soothsayer, among those who were killed by them. 23 And the border of the children of Reuben was the bank of the Jordan. This was the inheritance of the children of Reuben according to their families, the cities and their villages.
-Joshua 13:15-23 (NKJV)
Then Moses went up from the plains of Moab to Mount Nebo, to the top of Pisgah, which is across from Jericho. And the Lord showed him all the land of Gilead as far as Dan,
-Deuteronomy 34:1 (NKJV)
(Map via Nelson's Complete Book of Bible Maps and Charts, 3rd Edition, 2010.)
Here is a closeup of the land of Reuben:
(Map via Nelson's Complete Book of Bible Maps and Charts, 3rd Edition, 2010.)
So there was still plenty of land to see in all directions that would belong to Israel. Additionally Moab would be subjected to king David and his kingdom, along with that of Solomon's, would extend all the way from the Euphrates to the river of Egypt (2 Samuel 8 & 1 Kings 4:20-21, 24):
(Map via http://web.ccbce.com/multimedia/Atlas/)
All this fulfilled God's promise to Abram:
On the same day the Lord made a covenant with Abram, saying:
“To your descendants I have given this land, from the river of Egypt to the great river, the River Euphrates—”
-Genesis 15:18 (NKJV)