Click on the top left corner on the reward tab, you'll see those free sidebet. If you click on it then it will redirect you the specific slots and afterward, you can play on the slot as usual without really having to do anything. Its pretty simple, as long as the payout is higher then 2x excluding your stake then your sidebet won so you'll get extra winning

Not sure why it's not working for you but it works fine for me.
If you are accessing the website from your mobile then the sidebet tab will appear for a moment then you will be redirected to the game. Maybe you didn't notice it.
You can switch to the desktop version so the game won't be open in full screen then you can place your side bets.

Isn't slots a bit risky to play from a mobile device? It seems to me that it is somewhat risky because very good phone resources are needed so that the bets do not fail.

-snip