When advancing to higher leagues, you unlock new tracks to race on in Duels. By default, the tracks of Great Britain and Austria are already available for you to race on, since these are the only tracks available in League 8 (the league all players begin in).

If you are promoted for the very first time to League 5, you will unlock the tracks of Hungary and Canada in Duel mode. To see the rewards and tracks you can unlock through the leagues, tap on the "Play" button on the Main Menu, and then, on the icon indicating your league above the "DUEL" tile. There, you will see the League Progression with icons of the tracks you unlock as you reach higher leagues.