You are always accumulating FOX back as you trade (yes even on FREE trades). To view the amount of FOX you've earned, make you sure you have FOX Token toggled on in your mobile app.
From your Dashboard, click into FOX Token and see how much you've accumulated! Keep in mind you need at least 20 FOX in order to Claim it to your wallet.
If you would like to learn more about FOX please see the FOX website here!