You can install mysql, mongodb, GoogleCloud or AWS NPM packages, and access you remote DB from your backend code. To do that, click the plus sign (+) next to the “backend” code on the left panel, and choose “Install NPM package”.
You can then import it from your backend code and use it as you want.
If you’re missing a specific NPM for your DB, you can ask Wix to add it from within the NPM install dialog.