Sandbox search endpoints return no data

The sandbox environment does not return any data for either /search or /advanced-search endpoints.


Returns 0 results. On live it returns 485481 results.

According to the documentation (API testing):

the search API responds with data from live due to processing limitations

Is there anything else that needs to be done in the sandbox to get it working, such as populating with test data (the above comment from the documentation would lead me to believe that it will just work similar to the live environment)?

Any help appreciated.