No funds are stored on your account at The Sandbox.
Your funds and NFTs are already stored in whichever wallet that you connected to your account on The Sandbox when you registered. There is no such thing as a "Sandbox wallet".
The $SAND user interface on our website is simply showing you how much $SAND is in the wallet that you connected to the account.
Where are my funds then?
Your funds and NFTs are in whichever wallet that you connected to your account when you registered. It is your sole responsibility to remember and keep safe your cryptocurrency wallet credentials and the tokens inside that wallet.