You need to set up the conversion API between FB and Ringba first. This will allow Ringba to fire off conversion data back to Facebook. Then you need to be sure that conversion is attached to the ad account you are using for the ads. Then create a custom audience around that conversion pixel. Then you can run a conversion campaign based on the conversion from that conversion matrix by using the custom audience.
Here are some articles that will provide you with detailed instructions:
https://support.ringba.com/en-us/article/facebook-offline-events-integration-js-version-143k4xq/
https://support.ringba.com/en-us/article/facebook-offline-events-integration-call-only-ads-1751df2/