Hi,
We are not familiar with IDX or the various IDX plugins and themes that exist. We do know that if you go to SEO-XML Sitemaps-Post Types you may be able to add IDX pages into the sitemap. If no such options exist then we suggest two actions:
1. Speak with your IDX plugin/theme developer and ask for integration with the Yoast plugin. They may need to use an external plugin filter. This guide explains more: https://kb.yoast.com/kb/add-external-sitemap-to-index/
2.Use custom code to develop the ability. Though, we can’t offer support on custom code (needed to change (core) features of our plugin). Since we don’t want to take the risk that we break your website by suggesting incorrect or incomplete code, we cannot advise you on how to make such changes. Maybe someone watching these forums can assist you further, but if your topic is inactive for 2 weeks, we’ll mark it as resolved to keep the overview.
