I can't find a clear timeline of the events that preceded the conquest but in a short recap:
Gad and Reuben asked to settle in Transjordan
Moses agrees and offers them to build cities(?) and fences: "בְּנוּ־לָכֶם עָרִים לְטַפְּכֶם וּגְדֵרֹת לְצֹנַאֲכֶם וְהַיֹּצֵא מִפִּיכֶם תַּעֲשׂוּ׃" - "Build towns for your children and sheepfolds for your flocks, but do what you have promised." (Num 32,24)
They built numerous cities: "וַיִּבְנוּ בְנֵי־גָד אֶת־דִּיבֹן וְאֶת־עֲטָרֹת וְאֵת עֲרֹעֵר׃ וְאֶת־עַטְרֹת שׁוֹפָן וְאֶת־יַעְזֵר וְיָגְבֳּהָה׃ וְאֶת־בֵּית נִמְרָה וְאֶת־בֵּית הָרָן עָרֵי מִבְצָר וְגִדְרֹת צֹאן׃", etc (ibid 34-42)
Those tribes totaled over 100K families with a population of probably reaching the current size of Jerusalem (600K, given 4 kids per household).
Completion of those cities must have preceded the conquest, which occurred only a couple of months later.
What is called "a city" that they built? And how did they build those cities so fast?