You can play runs, build your Depot and do most things without the internet. Some things need a connection: leaderboards and leagues, racing other players, the daily login reward, the cloud backup of your progress, and purchases.
If you are online but the game says offline
- Check another app can reach the internet.
- Turn off any VPN, and try Wi-Fi instead of mobile data (or the other way round).
- Make sure your phone's date and time are set automatically.
- Close the game fully and open it again.
The game checks for the connection again every minute, and anything waiting (like your scores) is sent once it's back.