Once you pres play you should see a download bar in the top right of screen. It should finish by the time you get into game. Is this not the case? Can you not press play while the extra download is going?

If your can, please provide feedback in the settings menu and I can DM you for your Epic ID and look up the feedback.

boss_show

I can press play but it downloads in the back grounds and eats up my data, plus the game lags when it downloads at the back ground

Is it not downloading via WiFi or is that unavailable? Preferably you would download over WiFi, have the extra assets downloaded, and play without issue.

But playing while those assets are downloading would cause some performance issues such as hitching or stuttering.