Update 13, which was released on the 2nd of July, introduces changes on the League rewards system.
There are now rewards that you can unlock and then collect manually upon reaching specific Cup Points thresholds.
To see these rewards and Cup Points thresholds that you have to reach in order to collect these: tap on the number of Cup Points you have on the Duel mode screen.
Keep in mind that if you didn't claim your rewards after reaching a threshold, you will still be able to collect this threshold's rewards even if you have lost Duels and now have less Cup Points compared to the amount of Cup Points required to unlock the threshold's rewards.
Finally, the Elite League rewards can still be claimed after each monthly Elite League reset (which occurs on the first Monday of each month at 10:00 AM UTC). Therefore, do not forget to collect your Elite League rewards before the next monthly Elite League reset!