Just to clarify for people finding this topic later, with InstantSearch, you can only access the data that was indexed in Algolia in the first place, it doesn’t communicate with your database.
You can fetch any attribute that was indexed. Everything you can see in your Algolia dashboard is retrieved by instantsearch. If you want to use an attribute that is not indexed by default, you’ll have to index them first.
We have a tutorial to help index custom attribute, you should only have to modify a few lines to retrieve the correct data from WordPress: https://community.algolia.com/wordpress/custom-attributes.html
Please let me know if that help.