John Gill Commentary Numbers 21:13

"From thence they journeyed, and encamped on the other side of the Arnon, which is in the wilderness, that cometh out of the border of the Amorites: for the Arnon is the border of Moab, between Moab and the Amorites." — Numbers 21:13 (ASV)

From there they removed, and pitched on the other side of
Arnon
A river on the borders of Moab:

which is in the wilderness that comes out of the coasts of the
Amorites ;
according to Jarchi, they went round the land of Moab, all to the south and east, and came not into the border of Moab, as Jephthah said, (Judges 11:18) but before they came here they had a station at Almondiblathaim, (Numbers 33:46Numbers 33:47)

for Arnon is the border of Moab, between Moab and the Amorites ;
a river which divided these two countries, and bounded them; and Moses is the more particular in this account, to show that the Israelites took nothing from the Moabites, but what the Amorites had taken from them, they being charged not to distress the Moabites and Ammonites, (Deuteronomy 2:9Deuteronomy 2:19) , see Jephthah's defence, (Judges 11:15–27) .