Ai is not et-Tell!
If the location of Ai was et-Tell, Ai would have been a heap of ruins for centuries already in the period of the Conquest and would not have been conquered by Joshua, while Joshua 8 tells us in detail about the Conquest of Ai by the Israelites. Is Ai correctly identified with et-Tell? The answer appears to depend on the location of Bethel. It has long been accepted that the location of Bethel was the village of Beitin. Various passages in the Bible and reports from pilgrims show that this identification is not correct. Therefore the identification of Ai with et-Tell is incorrect too.
Cities did exist east of the Jordan in c. 1400 B.C.!
From his archaeological surveys east of the Jordan Nelson Glueck concluded in the thirties that the kingdoms of Sihon and Og did not exist before the end of the 13th century B.C., because there were no cities in the area between c. 1900 and c. 1250 B.C.. The results of recent excavations east of the Jordan have completely disproved Glueck's conclusions.
Ample evidence has been found of Late Bronze cities both in north- and in central Transjordan. No reason is left, therefore, not to date the Conquest by the Israelites in the 14th century B.C..