John Gill Commentary Deuteronomy 2:18

"Thou art this day to pass over Ar, the border of Moab:" — Deuteronomy 2:18 (ASV)

You are to pass over through Ar
That is, over the river Arnon, by the city Ar of Moab, which was situated by it; see (Deuteronomy 2:9) and so Moses and the people of Israel were to pass along by that,

and by the coast of Moab ;
for they were not admitted to enter the land and pass through it; only to travel on the borders of it, and that they were to begin to do this day; the day the Lord spoke to Moses.