John Gill Commentary Joshua 13:16

"And their border was from Aroer, that is on the edge of the valley of the Arnon, and the city that is in the middle of the valley, and all the plain by Medeba;" — Joshua 13:16 (ASV)

And their coast was from Aroer that [is] on the bank of the
river Arnon
As the country of Sihon is described, (Joshua 13:9) ; from which it appears that it was his country which was given to Reuben, though not all of it:

and the city that [is] in the midst of the river ;
(See Gill on Joshua 13:9);

and all the plain by Medeba ;
which reached unto Dibon, (Joshua 13:9) .